Re-export more rustc_span::symbol things from rustc_span.

`rustc_span::symbol` defines some things that are re-exported from
`rustc_span`, such as `Symbol` and `sym`. But it doesn't re-export some
closely related things such as `Ident` and `kw`. So you can do `use
rustc_span::{Symbol, sym}` but you have to do `use
rustc_span::symbol::{Ident, kw}`, which is inconsistent for no good
reason.

This commit re-exports `Ident`, `kw`, and `MacroRulesNormalizedIdent`,
and changes many `rustc_span::symbol::` qualifiers in `compiler/` to
`rustc_span::`. This is a 200+ net line of code reduction, mostly
because many files with two `use rustc_span` items can be reduced to
one.

compiler/rustc_target/src/asm/mod.rs

@@ -35,7 +35,7 @@ macro_rules! def_reg_class {
         impl $arch_regclass {
             pub fn name(self) -> rustc_span::Symbol {
                 match self {
-                    $(Self::$class => rustc_span::symbol::sym::$class,)*
+                    $(Self::$class => rustc_span::sym::$class,)*
                 }
             }
 
@@ -511,7 +511,7 @@ impl InlineAsmRegClass {
             Self::Msp430(r) => r.name(),
             Self::M68k(r) => r.name(),
             Self::CSKY(r) => r.name(),
-            Self::Err => rustc_span::symbol::sym::reg,
+            Self::Err => rustc_span::sym::reg,
         }
     }
 

compiler/rustc_target/src/spec/mod.rs

@@ -46,7 +46,7 @@ use rustc_data_structures::fx::FxHashSet;
 use rustc_fs_util::try_canonicalize;
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
 use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
-use rustc_span::symbol::{Symbol, kw, sym};
+use rustc_span::{Symbol, kw, sym};
 use serde_json::Value;
 use tracing::debug;
 

compiler/rustc_target/src/target_features.rs

@@ -3,7 +3,7 @@
 //! and Rust adds some features that do not correspond to LLVM features at all.
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
-use rustc_span::symbol::{Symbol, sym};
+use rustc_span::{Symbol, sym};
 
 use crate::spec::Target;