[Clang][Lex] Fix parsing of nested requirement to prevent flowing off the end...

[Clang][Lex] Fix parsing of nested requirement to prevent flowing off the end of token stream (#73691)

Currently when parsing a nested requirement we attempt to balance parens
if we have a parameter list. This will fail in some cases of ill-formed
code and keep going until we fall off the token stream and crash. This
fixes the hand parsing by using SkipUntil which will properly flag if we
don't find the expected tokens.

Fixes: https://github.com/llvm/llvm-project/issues/73112