codegen_llvm: set DW_AT_accessibility

Sets the accessibility of types and fields in DWARF using
`DW_AT_accessibility` attribute.

`DW_AT_accessibility` (public/protected/private) isn't exactly right for
Rust,  but neither is `DW_AT_visibility` (local/exported/qualified), and
there's no way to set `DW_AT_visbility` in LLVM's API.

Signed-off-by: David Wood <david@davidtw.co>

tests/codegen/debug-accessibility/crate-struct.rs

@@ -0,0 +1,23 @@
+// compile-flags: -C debuginfo=2
+
+#![allow(dead_code)]
+
+// Checks that visibility information is present in the debuginfo for crate-visibility structs.
+
+mod module {
+    use std::hint::black_box;
+
+    pub(crate) struct CrateFooStruct {
+        x: u32,
+    }
+
+    // CHECK: {{!.*}} = !DICompositeType(tag: DW_TAG_structure_type, name: "CrateFooStruct"{{.*}}flags: DIFlagProtected{{.*}})
+
+    pub fn use_everything() {
+        black_box(CrateFooStruct { x: 2 });
+    }
+}
+
+fn main() {
+    module::use_everything();
+}