[flang][semantics] Reject DATA-style initializer on EXTERNAL/INTRINSIC (#222256)

An `entity-decl` carrying a legacy `/initialization/` (an extension) was silently
accepted, and the initializer dropped, when the name had already been
declared `EXTERNAL` or `INTRINSIC`:

```fortran
subroutine s
  external foo
  integer foo /1/   ! accepted, no initialization emitted
end subroutine
```

`DataChecker::Leave(const parser::EntityDecl &)` passed the value list to
`AccumulateDataInitializations` without checking the symbol's class, so the
initialization never reached the emitted code and no diagnostic was produced.
Reversing the two statements already errors, so the behaviour was also
order-dependent. For an intrinsic name that is not an unrestricted specific
function (`sum`), the accumulated value instead tripped
`CHECK(designator.has_value())` in `data-to-inits.cpp` and crashed the
compiler.

A procedure name is not a data-stmt-object (F2023 C880, R842), so reject it here.
Procedure pointer initialization is an accepted extension (8.6.7), so
`IsProcedurePointer` symbols still take the existing path.

Fixes #222168

Assisted-by: Qoder (AI assistant)