ARM ABI: passing illegal vector types as varargs.

We expand varargs in clang and the call site is handled in the back end, it is
hard to match exactly how illegal vectors are handled in the backend. Therefore,
we legalize the illegal vector types in clang:
if (Size <= 32), legalize to i32.
if (Size == 64), legalize to v2i32.
if (Size == 128), legalize to v4i32.
if (Size > 128), use indirect.

rdar://12439123

llvm-svn: 166043