[flang] USE-associated explicit INTRINSIC names (#76199)

The compiler doesn't USE-associate names of intrinsic procedures from
modules (in the absence of ONLY:), so that the associating scope doesn't
get populated with names of intrinsics that were used only in
declarations (e.g., SELECTED_REAL_KIND). A recent bug report (below)
shows that we should modify that policy in the case of names that appear
in explicit INTRINSIC attribute statements. The behaviors of other
Fortran compilers are not consistent and the requirements of the
standard are not clear; this fix follows the precedent set by gfortran
and nvfortran.

Fixes https://github.com/llvm/llvm-project/issues/72084.