The alias was only ever used on darwin and had some issues there,
and isn't used in practice much. Also fixes a problem with -mno-altivec
not turning off -maltivec.

Also add a diagnostic for faltivec/fno-altivec that directs users to use
maltivec options and include the altivec.h file explicitly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@298449 91177308-0d34-0410-b5e6-96231b3b80d8