[flang] Extension: unrestricted intrinsics as specifics in generics

At least one other Fortran compiler supports the use of unrestricted intrinsic
functions as specific procedures in generic interfaces, and the usage seems
to be both useful and unambiguous.  Support it with a portability warning.

Fixes llvm-test-suite/Fortran/gfortran/regression/pr95500.f90.

Differential Revision: https://reviews.llvm.org/D157333