Add builtin_elementwise_log

Add codegen for llvm log elementwise builtin
The log 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.
The new builtin is restricted to floating point types only.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D140489