[flang] Expose specific to generic intrinsic name mapping

The intrinsic lowering facility is based on the generic intrinsic names to avoid
duplicating implementations. Specific intrinsics call are re-written to call to
the generic versions by the front-end but this cannot be done when specific intrinsics
are passed as arguments (the rewrite would give illegal/ambiguous unparsed Fortran).
Solve the issue by making the specific to generic name mapping accessible to lowering
and can be later used to generate the unrestricted intrinsic functions.

Reviewed By: schweitz

Differential Revision: https://reviews.llvm.org/D84842