Cleanup result_of tests and fix issues with the C++03 result_of.

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.

llvm-svn: 239668