[flang] Fix declaration of module procedure subprograms

Names of subprograms declared with MODULE PROCEDURE in a submodule
were not found correctly. The fix is to separate the handling of
these from other subprograms. The subprogram being defined must have
been declared in the same module or an ancestor module/submodule.

Fixes flang-compiler/f18#709

Original-commit: flang-compiler/f18@80b635d34355d657a807e7f276b8d40d191bcef9
Reviewed-on: https://github.com/flang-compiler/f18/pull/710