Implement support for DWARF version 5.

DWARF version 5 brings a number of improvements over version 4. Quoting from
the announcement [1]:

> Version 5 incorporates improvements in many areas: better data compression,
> separation of debugging data from executable files, improved description of
> macros and source files, faster searching for symbols, improved debugging
> optimized code, as well as numerous improvements in functionality and
> performance.

On platforms where DWARF version 5 is supported (Linux, primarily), this commit
adds support for it behind a new `-Z dwarf-version=5` flag.

[1]: https://dwarfstd.org/Public_Review.php

compiler/rustc_target/src/spec/android_base.rs

@@ -3,7 +3,7 @@ use crate::spec::TargetOptions;
 pub fn opts() -> TargetOptions {
     let mut base = super::linux_base::opts();
     base.os = "android".into();
-    base.dwarf_version = Some(2);
+    base.default_dwarf_version = 2;
     base.position_independent_executables = true;
     base.has_thread_local = false;
     // This is for backward compatibility, see https://github.com/rust-lang/rust/issues/49867

compiler/rustc_target/src/spec/apple_base.rs

@@ -28,7 +28,7 @@ pub fn opts(os: &'static str) -> TargetOptions {
         executables: true,
         families: cvs!["unix"],
         is_like_osx: true,
-        dwarf_version: Some(2),
+        default_dwarf_version: 2,
         frame_pointer: FramePointer::Always,
         has_rpath: true,
         dll_suffix: ".dylib".into(),

compiler/rustc_target/src/spec/dragonfly_base.rs

@@ -9,7 +9,7 @@ pub fn opts() -> TargetOptions {
         has_rpath: true,
         position_independent_executables: true,
         relro_level: RelroLevel::Full,
-        dwarf_version: Some(2),
+        default_dwarf_version: 2,
         ..Default::default()
     }
 }

compiler/rustc_target/src/spec/freebsd_base.rs

@@ -10,7 +10,7 @@ pub fn opts() -> TargetOptions {
         position_independent_executables: true,
         relro_level: RelroLevel::Full,
         abi_return_struct_as_int: true,
-        dwarf_version: Some(2),
+        default_dwarf_version: 2,
         ..Default::default()
     }
 }

compiler/rustc_target/src/spec/mod.rs

@@ -1275,9 +1275,9 @@ pub struct TargetOptions {
     pub is_like_msvc: bool,
     /// Whether a target toolchain is like WASM.
     pub is_like_wasm: bool,
-    /// Version of DWARF to use if not using the default.
+    /// Default supported version of DWARF on this platform.
     /// Useful because some platforms (osx, bsd) only want up to DWARF2.
-    pub dwarf_version: Option<u32>,
+    pub default_dwarf_version: u32,
     /// Whether the linker support GNU-like arguments such as -O. Defaults to true.
     pub linker_is_gnu: bool,
     /// The MinGW toolchain has a known issue that prevents it from correctly
@@ -1539,7 +1539,7 @@ impl Default for TargetOptions {
             is_like_windows: false,
             is_like_msvc: false,
             is_like_wasm: false,
-            dwarf_version: None,
+            default_dwarf_version: 4,
             linker_is_gnu: true,
             allows_weak_linkage: true,
             has_rpath: false,
@@ -1778,13 +1778,13 @@ impl Target {
                     base.$key_name = s;
                 }
             } );
-            ($key_name:ident, Option<u32>) => ( {
+            ($key_name:ident, u32) => ( {
                 let name = (stringify!($key_name)).replace("_", "-");
                 if let Some(s) = obj.remove(&name).and_then(|b| b.as_u64()) {
                     if s < 1 || s > 5 {
                         return Err("Not a valid DWARF version number".into());
                     }
-                    base.$key_name = Some(s as u32);
+                    base.$key_name = s as u32;
                 }
             } );
             ($key_name:ident, Option<u64>) => ( {
@@ -2143,7 +2143,7 @@ impl Target {
         key!(is_like_windows, bool);
         key!(is_like_msvc, bool);
         key!(is_like_wasm, bool);
-        key!(dwarf_version, Option<u32>);
+        key!(default_dwarf_version, u32);
         key!(linker_is_gnu, bool);
         key!(allows_weak_linkage, bool);
         key!(has_rpath, bool);
@@ -2387,7 +2387,7 @@ impl ToJson for Target {
         target_option_val!(is_like_windows);
         target_option_val!(is_like_msvc);
         target_option_val!(is_like_wasm);
-        target_option_val!(dwarf_version);
+        target_option_val!(default_dwarf_version);
         target_option_val!(linker_is_gnu);
         target_option_val!(allows_weak_linkage);
         target_option_val!(has_rpath);

compiler/rustc_target/src/spec/netbsd_base.rs

@@ -11,7 +11,7 @@ pub fn opts() -> TargetOptions {
         position_independent_executables: true,
         relro_level: RelroLevel::Full,
         use_ctors_section: true,
-        dwarf_version: Some(2),
+        default_dwarf_version: 2,
         ..Default::default()
     }
 }

compiler/rustc_target/src/spec/openbsd_base.rs

@@ -11,7 +11,7 @@ pub fn opts() -> TargetOptions {
         position_independent_executables: true,
         frame_pointer: FramePointer::Always, // FIXME 43575: should be MayOmit...
         relro_level: RelroLevel::Full,
-        dwarf_version: Some(2),
+        default_dwarf_version: 2,
         ..Default::default()
     }
 }