[flang][debug] Debugger shows the wrong variable for same-named submodule entities

A variable that is private to a submodule is described in the debug information
as belonging to the submodule's ancestor module. Two submodules of the same
module may each declare a variable of the same name, and both then land in that
one module. A debugger cannot tell them apart and silently reports the value of
whichever it finds first, with no error or ambiguity warning.

## Reproducer

`repro.f90`:

```fortran
module m
interface
module subroutine one()
end subroutine
module subroutine two()
end subroutine
end interface
end module

submodule (m) s1
integer :: counter = 111
contains
module subroutine one()
print *, counter
end subroutine
end submodule

submodule (m) s2
integer :: counter = 222
contains
module subroutine two()
print *, counter
end subroutine
end submodule

program p
use m
call one()
call two()
end program p
```

## Steps to reproduce

```
flang -g -O0 repro.f90 -o repro
gdb -q -batch -ex "break two" -ex run -ex "print counter" ./repro
```

## Expected

`two` is defined in submodule `s2`, whose `counter` is 222, so the debugger
should print 222. Running the program confirms the value is 222: the program
prints `111` then `222`.

```
$1 = 222
```

## Actual

The debugger prints the `counter` of submodule `s1` instead.

```
Breakpoint 1, m::two () at repro.f90:22
$1 = 111
```

Breaking in `one` also prints 111, which happens to be correct there, so the
same expression silently means different things depending on nothing the user
can see.