[PowerPC][AIX] Make `__vector [un]signed long` an error

The semantics associated with `__vector [un]signed long` are neither
consistently specified nor consistently implemented.

The IBM XL compilers on AIX traditionally treated these as deprecated
aliases for the corresponding `__vector int` type in both 32-bit and
64-bit modes. The newer, Clang-based, IBM XL compilers on AIX make usage
of the previously deprecated types an error. This is also consistent
with IBM XL C/C++ for Linux on Power (on little endian distributions).

In line with the above, this patch upgrades (on AIX) the deprecation of
`__vector long` to become removal.

Reviewed By: ZarkoCA

Differential Revision: https://reviews.llvm.org/D89443