[flang] Restore error status for many indistinguishable specifics (#79927)

A recent patch to allow pFUnit to compile softened the diagnostic about
indistinguishable specific procedures to a portability warning. It turns
out that this was overkill -- for specific procedures containing no
optional or unlimited polymorphic dummy data arguments, a diagnosis of
"indistinguishable" can still be a hard error.

So adjust the analysis to be tri-state: two procedures are either
definitely distinguishable, definitely indistinguishable without
optionals or unlimited polymorphics, or indeterminate. Emit errors as
before for the definitely indistinguishable cases; continue to emit
portability warnings for the indeterminate cases.

When this patch is merged, all but one of the dozen or so tests that I
disabled in llvm-test-suite can be re-enabled.