When "delayed parsing" C++ default arguments, if there is an error, there may be...

When "delayed parsing" C++ default arguments, if there is an error, there may be tokens left in the token stream
that will interfere (they will be parsed as if they are after the class' '}') and a crash will occur because
the CachedTokens that holds them will be deleted while the lexer is still using them.

Make sure that the tokens of default args are removed from the token stream.
Fixes PR6647.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@99939 91177308-0d34-0410-b5e6-96231b3b80d8