Looking for a function-like macro’s opening parenthesis

Function-like macros only expand when immediately followed by a parenthesis. To do this cpplib needs to temporarily disable macros and read the next token. Unfortunately, because of spacing issues (see Token Spacing), there can be fake padding tokens in-between, and if the next real token is not a parenthesis cpplib needs to be able to back up that one token as well as retain the information in any intervening padding tokens.

Backing up more than one token when macros are involved is not permitted by cpplib, because in general it might involve issues like restoring popped contexts onto the context stack, which are too hard. Instead, searching for the parenthesis is handled by a special function, funlike_invocation_p, which remembers padding information as it reads tokens. If the next real token is not an opening parenthesis, it backs up that one token, and then pushes an extra context just containing the padding information if necessary.