[flang] Turn "error" cases into warning for "indistinguishable" specific procedures (#79621)

When a generic procedure interface, either declared or the result of
    merging two use-associated generics, has two specific procedures
    that are not distinguishable according to the rules in F'2023
    subclause 15.4.3.4.5, emit a portability warning rather than a
    hard error message.  The rules in that subclause are not adequate
    to detect pairs of specific procedures that admit an ambiguous
    reference, as demonstrated by a case that arose in pFUnit.  Further,
    these distinguishability checks, even if sufficient to the task
    of detecting pairs of specifics capable of ambiguous references,
    should only apply to pairs where *every* reference would have to
    be ambiguous -- and this can and is validated at every reference
    anyway.  Last, only XLF enforces these incomplete and needless
    distinguishability rules -- every other compiler seems to just
    check that each procedure reference resolves to exactly one
    specific procedure.
    
    If the standard were to complete lose subclause 15.4.3.4.5 and
    its related note (C.11.6) -- which admits that the rules are
    incomplete! -- and simply require that each generic procedure
    reference resolve unambiguously to exactly one specific, nobody
    would miss them.  This patch changes this compiler to give them
    lip service when requested, but they are now otherwise ignored.