[flang] Change handling of BIND attributes

Previously `BIND(C, NAME="...")` caused `bindName` to be set but not
the `BIND` attribute. This made writing the `.mod` file easier, but it
makes it hard to ask: does this symbol have a C binding.

This change causes the `BIND` attribute to be set whenever there is
`BIND(C)` in the source, regardless of whether there is a name or not.
`bindName` is only set when the `BIND` attribute is also set.

Original-commit: flang-compiler/f18@5a6706ffcbb96c4439a6af5a1f739ad4e7b8c00a
Reviewed-on: https://github.com/flang-compiler/f18/pull/298
Tree-same-pre-rewrite: false