clang: Add elementwise pow builtin

Add codegen for llvm pow elementwise builtin
The pow elementwise builtin is necessary for HLSL codegen.
Tests were added to make sure that the expected errors are encountered when these functions are given inputs of incompatible types, or too many inputs.
The new builtin is restricted to floating point types only.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D153310