[Clang] Add integer mul reduction builtin

Similar to the existing bitwise reduction builtins, this lowers to a llvm.vector.reduce.mul intrinsic call.

For other reductions, we've tried to share builtins for float/integer vectors, but the fmul reduction intrinsic also take a starting value argument and can either do unordered or serialized, but not reduction-trees as specified for the builtins. However we address fmul support this shouldn't affect the integer case.

Differential Revision: https://reviews.llvm.org/D117829