The two main fixes this patch contains are:
- use __identity_t instead of common_type. common_type was used as an
identity metafunction but the decay resulted in incorrect results.
- Pointers to free functions were not counted as functions. Remove the pointer
before checking if a type is a function.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@239668 91177308-0d34-0410-b5e6-96231b3b80d8
