[Clang] Fix a crash when parsing an invalid decltype (#148798)

We would try to exact an annotated token before checking if it was valid, leading to a crash when `decltype` was the only token that was parsed (which can happen in the absense of opening paren) Fixes #114815

Author: cor3ntin

clang/lib/Parse/ParseDeclCXX.cpp

@@ -1133,13 +1133,6 @@ void Parser::AnnotateExistingDecltypeSpecifier(const DeclSpec &DS,
  // make sure we have a token we can turn into an annotation token
  if (PP.isBacktrackEnabled()) {
  PP.RevertCachedTokens(1);
- if (DS.getTypeSpecType() == TST_error) {
- // We encountered an error in parsing 'decltype(...)' so lets annotate all
- // the tokens in the backtracking cache - that we likely had to skip over
- // to get to a token that allows us to resume parsing, such as a
- // semi-colon.
- EndLoc = PP.getLastCachedTokenLocation();
- }
  } else
  PP.EnterToken(Tok, /*IsReinject*/ true);
 

clang/test/Parser/gh114815.cpp

@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -verify %s -std=c++11 -fsyntax-only
+
+#define ID(X) X
+extern int ID(decltype);
+// expected-error@-1 {{expected '(' after 'decltype'}} \
+// expected-error@-1 {{expected unqualified-id}}