[flang] Distinguish intrinsic module USE in module files; correct search paths

In the USE statements that f18 emits to module files, ensure that symbols
from intrinsic modules are marked as such on their USE statements.  And
ensure that the current working directory (".") cannot override the intrinsic
module search path when trying to locate an intrinsic module.

Differential Revision: https://reviews.llvm.org/D127019