Headers: Define __INT64_TYPE__ in terms of getInt64Type(), which isn't always

Headers: Define __INT64_TYPE__ in terms of getInt64Type(), which isn't always
'long'. The practical upshot is so that the uint64_t we define in our stdint.h
ends up being compatible with that defined by gcc (at least on Darwin), which
otherwise could lead to type incompatibilities with other system headers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@107255 91177308-0d34-0410-b5e6-96231b3b80d8