[flang] Warn when a submodule subprogram hides a like-named external-interface body from its parent module

PR #197173 added a portability warning for a submodule subprogram that is
missing the `MODULE` prefix required to implement a *separate module procedure*
interface from its parent. That PR was deliberately scoped to separate module
procedure interfaces. The scenario originally reported in #197171 uses a plain
(non-`MODULE`) interface body — so although #197171 is now closed, this shape
still produces no diagnostic:

```fortran
module m
interface
subroutine inside_one()
end subroutine
end interface
end module

submodule (m) s
contains
subroutine inside_one()
end subroutine
end submodule
```

```console
$ flang -fsyntax-only -pedantic t.f90 # upstream main 62b7ae5dc327
$ # no diagnostic
```

Here the submodule's `inside_one` is an ordinary submodule-local subprogram
that hides the external interface; it neither implements the interface nor is
callable from outside. A diagnostic would be useful, but it needs different
wording than #197173's: suggesting a `MODULE` prefix is wrong for this shape —
following it is a hard error, since `inside_one` was not declared a separate
module procedure:

```console
$ flang -fsyntax-only -pedantic t-with-module-prefix.f90 # same, with MODULE prefix added
error: 'inside_one' was not declared a separate module procedure
```

Something like "Subprogram 'x' in this submodule hides the external interface
'x' from its parent module and cannot implement it" would fit.

Reproduce the missing diagnostic with t.f90 using `flang -fsyntax-only -pedantic`, then compare it with t-with-module-prefix.f90 and its existing error. Trace the handling of submodule-local subprograms and external interfaces; done means the plain-interface case emits a warning with wording that does not suggest adding `MODULE`.

Written by the indexing model from the issue text.