[flang] Fix bug resolving internal and module functions

When analyzing a function call in an expression we weren't properly
recognizing a ProcedureDesignator that was the name of an internal
or module function, i.e. a symbol with SubprogramDetails.

The fix is to add IsProcedure to identify symbols that correspond
to procedures. IsFunction and GetType also need to be extended to
handle this case.

Fixes flang-compiler/f18#391.

Original-commit: flang-compiler/f18@f165f8d38ce0cefd16e0f35644e8fdcbe92ed6de
Reviewed-on: https://github.com/flang-compiler/f18/pull/417
Tree-same-pre-rewrite: false