[Flang] Lower the exp, log, log10 intrinsics

The intrinsic computes the exponent, log real and complex numbers and
log10 for real numbers. By default they are lowered to runtime calls to
libpgmath. kind=10 and 16 are not supported. With the llvm option, it
can be lowered to llvm intrinsics (not all types .eg. complex are
supported for llvm lowering).

This is part of the upstreaming effort from the fir-dev branch in [1].
[1] https://github.com/flang-compiler/f18-llvm-project

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D122132



Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>
Co-authored-by: William S Moses <gh@wsmoses.com>