[flang] Clean up ISO_FORTRAN_ENV, fix NUMERIC_STORAGE_SIZE (#87566)

Address TODOs in the intrinsic module ISO_FORTRAN_ENV, and extend the
implementation of NUMERIC_STORAGE_SIZE so that the calculation of its
value is deferred until it is needed so that the effects of
-fdefault-integer-8 or -fdefault-real-8 are reflected. Emit a warning
when NUMERIC_STORAGE_SIZE is used from the module file and the default
integer and real sizes do not match.

Fixes https://github.com/llvm/llvm-project/issues/87476.