[flang] Fix bug with statement functions

When the name of a statement function was previously declared, we
weren't correctly recognizing it as a statement function. E.g.
```
integer :: f, i
f(i) = i + 1
```

`f` was entered in the symbol table with `EntityDetails` and the
`parser::Name` on the first line was resolved to that symbol.
On the second line we replaced the symbol for `f` in the scope
with a subprogram symbol, but that didn't change the symbol in
the first `parser::Name`.

The fix requires:
1. don't erase the original symbol for `f`, just replace its details
2. when we erase the symbol for `f` in the subprogram scope, don't
   unresolve it

Original-commit: flang-compiler/f18@31212686ea357503840c0562853c6977bfc1ffe5