[Clang] Add option to handle behaviour of vector bool/vector pixel.

Added the option `-altivec-src-compat=[mixed,gcc,xl]`. The default at this time is `mixed`.

The default behavior for clang is for all vector compares to return a scalar unless the vectors being
compared are vector bool or vector pixel. In that case the compare returns a
vector. With the gcc case all vector compares return vectors and in the xl case
all vector compares return scalars.

This patch does not change the default behavior of clang.

This option will be used in future patches to implement behaviour compatibility for the vector bool/pixel types.

Reviewed By: bmahjour

Differential Revision: https://reviews.llvm.org/D103615