[flang] Accept unambiguous USE name clashes

When, due to one or more USE associations, possibly with renaming,
a symbol conflicts with another of the same name in the same scope,
don't raise an error if both symbols resolve to the same intrinsic
procedure or to the same non-generic external procedure interface --
the usage is unambiguous and safe, and (14.2.2 p8) standard.

(Generic interfaces already work by way of combining their sets of
specific procedures.)

Differential Revision: https://reviews.llvm.org/D132682