Lex: Require that '#' be followed by a macro parameter name when preceded by '##'

After lexing a '##', we would look ahead and check to see if it was
followed by '__VA_ARGS__'.  After doing so, we would then go ahead and
lex the token.

However we would fail in the case where the '##' was followed by a '#'
followed by an identifier because we would have lexed the '#' separately
from the identifier, bypassing our parameter validation logic.

Instead, lex the tokens coming after the '##' later.

This fixes PR17804.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194059 91177308-0d34-0410-b5e6-96231b3b80d8