[flang] Generic name resolution in expression analysis

Implement the basics of resolving generic names in expressions.

`ExpressionAnalyzer::ResolveGeneric` maps the symbol for a generic
name to the specific procedure appropriate for the actual arguments.
Extract `CheckExplicitInterface` out of `CheckArguments` so that it
can be tried for each specific procedure of the generic as part of
the test to see which is compatible.

Note that it may be there is an elemental and non-elemental specific
procedure that is compatible with the actual arguments. In that case
the generic is resolved to the non-elemental one.

Test this by using generic functions in specification expressions
that must be written to module files. Verify how the generics were
resolved by looking at the generated `.mod` files.

There is more work to be done in this area: the passed-object dummy
argument is not considered and in some cases generated module files
are not correct.

Original-commit: flang-compiler/f18@50e458045a3de42bd60515956e09f0f2d654ae1e
Reviewed-on: https://github.com/flang-compiler/f18/pull/778