[flang] Upstreaming intrinsic call lowering.

This module implements the lowering of Fortran intrinsics to the
corresponding calls in support libraries (the Fortran runtime, math
libraries, etc.)

This revision is a tad larger because there are a large number of Fortran
intrinsics and this adds lowering for a fair number of them.

Differential revision: https://reviews.llvm.org/D83355