Extend comments for problems and future directions.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@188035 91177308-0d34-0410-b5e6-96231b3b80d8
Fix false error message for function-style macro instance used as arguments in other function-style macros instances, and add test for it.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@188036 91177308-0d34-0410-b5e6-96231b3b80d8