Substantially restructure function-like macro argument parsing.

Highlights: PP::isNextPPTokenLParen() no longer eats the ( 
when present.  We now simplify slightly the logic parsing
macro arguments.  We now handle PR3937 and other related cases
correctly.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@69411 91177308-0d34-0410-b5e6-96231b3b80d8