[flang] Cherry picking commits from #PR275 and simplifying it

Commit was: Draft of elemental intrinsics function folding

Simplification was made in order to solve conflicts and
to prepare for commit of host math library indirection.

Conflicts:
	lib/evaluate/fold.cc

Original-commit: flang-compiler/f18@45678bc44c796509a0d70a079e3912b11ebea2ab
Tree-same-pre-rewrite: false