[Clang] Add integer add reduction builtin
Similar to the existing bitwise reduction builtins, this lowers to a llvm.vector.reduce.add intrinsic call.
For other reductions, we've tried to share builtins for float/integer vectors, but the fadd reduction intrinsics 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 fadd support this shouldn't affect the integer case.
(Split off from D117829)
Differential Revision: https://reviews.llvm.org/D124741