Rearrange TokenTreesReader::parse_token_tree.

`parse_token_tree` is basically a match with four arms: `Eof`,
`OpenDelim`, `CloseDelim`, and "other". It has two call sites, and at
each call site one of the arms is unreachable. It's also not inlined.

This commit removes `parse_token_tree` by splitting it into four
functions and inlining them. This avoids some repeated conditional
tests and also some non-inlined function calls on the hot path.

compiler/rustc_errors/src/lib.rs

@@ -63,7 +63,8 @@ pub mod translation;
 pub use diagnostic_builder::IntoDiagnostic;
 pub use snippet::Style;
 
-pub type PResult<'a, T> = Result<T, DiagnosticBuilder<'a, ErrorGuaranteed>>;
+pub type PErr<'a> = DiagnosticBuilder<'a, ErrorGuaranteed>;
+pub type PResult<'a, T> = Result<T, PErr<'a>>;
 
 // `PResult` is used a lot. Make sure it doesn't unintentionally get bigger.
 // (See also the comment on `DiagnosticBuilder`'s `diagnostic` field.)