Fix this:

With this snippet:
  void f(a::b);

An assert is hit:
Assertion failed: CachedTokens[CachedLexPos-1].getLocation() == Tok.getAnnotationEndLoc() && "The annotation should be until the most recent cached token", file ..\..\lib\Lex\PPCaching.cpp, line 98

Introduce Preprocessor::RevertCachedTokens that reverts a specific number of tokens when backtracking is enabled.

llvm-svn: 59636