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_errors/src/diagnostic_impls.rs

@@ -9,9 +9,8 @@ use rustc_abi::TargetDataLayoutErrors;
 use rustc_ast::util::parser::ExprPrecedence;
 use rustc_ast_pretty::pprust;
 use rustc_macros::Subdiagnostic;
-use rustc_span::Span;
 use rustc_span::edition::Edition;
-use rustc_span::symbol::{Ident, MacroRulesNormalizedIdent, Symbol};
+use rustc_span::{Ident, MacroRulesNormalizedIdent, Span, Symbol};
 use rustc_target::spec::{PanicStrategy, SplitDebuginfo, StackProtector, TargetTuple};
 use rustc_type_ir::{ClosureKind, FloatTy};
 use {rustc_ast as ast, rustc_hir as hir};