Rename DiagnosticBuilder as Diag.

Much better!

Note that this involves renaming (and updating the value of)
`DIAGNOSTIC_BUILDER` in clippy.

src/parse/parser.rs

@@ -3,7 +3,7 @@
 
 use rustc_ast::token::TokenKind;
 use rustc_ast::{ast, attr, ptr};
-use rustc_errors::DiagnosticBuilder;
+use rustc_errors::Diag;
 use rustc_parse::{new_parser_from_file, parser::Parser as RawParser};
 use rustc_span::{sym, Span};
 use thin_vec::ThinVec;
@@ -65,7 +65,7 @@ pub(crate) fn build(self) -> Result<Parser<'a>, ParserError> {
     fn parser(
         sess: &'a rustc_session::parse::ParseSess,
         input: Input,
-    ) -> Result<rustc_parse::parser::Parser<'a>, Option<Vec<DiagnosticBuilder<'a>>>> {
+    ) -> Result<rustc_parse::parser::Parser<'a>, Option<Vec<Diag<'a>>>> {
         match input {
             Input::File(ref file) => catch_unwind(AssertUnwindSafe(move || {
                 new_parser_from_file(sess, file, None)

src/parse/session.rs

@@ -6,7 +6,7 @@
 use rustc_errors::emitter::{DynEmitter, Emitter, HumanEmitter};
 use rustc_errors::translation::Translate;
 use rustc_errors::{
-    ColorConfig, DiagCtxt, DiagInner, DiagnosticBuilder, ErrorGuaranteed, Level as DiagnosticLevel,
+    ColorConfig, Diag, DiagCtxt, DiagInner, ErrorGuaranteed, Level as DiagnosticLevel,
 };
 use rustc_session::parse::ParseSess as RawParseSess;
 use rustc_span::{
@@ -300,7 +300,7 @@ pub(crate) fn get_original_snippet(&self, file_name: &FileName) -> Option<Lrc<St
 
 // Methods that should be restricted within the parse module.
 impl ParseSess {
-    pub(super) fn emit_diagnostics(&self, diagnostics: Vec<DiagnosticBuilder<'_>>) {
+    pub(super) fn emit_diagnostics(&self, diagnostics: Vec<Diag<'_>>) {
         for diagnostic in diagnostics {
             diagnostic.emit();
         }