Rollup merge of #60348 - agnxy:refactor-parser, r=petrochenkov

move some functions from parser.rs to diagostics.rs

Starting with a few functions mentioned in https://github.com/rust-lang/rust/issues/60015#issuecomment-484259773. We might refactor parser.rs further in subsequent changes.
r? @petrochenkov

src/libsyntax/parse/mod.rs

@@ -30,6 +30,7 @@ pub mod parser;
 pub mod lexer;
 pub mod token;
 pub mod attr;
+pub mod diagnostics;
 
 pub mod classify;