Basic: make `int_least64_t` and `int_fast64_t` match on Darwin

The Darwin targets use `int64_t` and `uint64_t` to define the `int_least64_t`
and `int_fast64_t` types.  The underlying type is actually a `long long`.  Match
the types to allow the printf specifiers to work properly and have the compiler
vended macros match the implementation on the target.

llvm-svn: 348939