[AVX512] Fix return types on __builtin_ia32_gather3XivXdi builtins

The return types on the AVX512 __builtin_ia32_gather3XivXdi builtins are incorrect. The return type should match the type of the pass through vector.

Differential Revision: https://reviews.llvm.org/D24785

-This line, and those below, will be ignored--

M    include/clang/Basic/BuiltinsX86.def

llvm-svn: 282082