The time when -faltivec (or, on clang only, -maltivec) will magically

The time when -faltivec (or, on clang only, -maltivec) will magically
include altivec.h has come and gone.

Rationale: This causes modules, rewrite-includes, etc to be sad and
people should just include altivec.h in their source.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@264235 91177308-0d34-0410-b5e6-96231b3b80d8