[flang] Allow for submodule override of module procedure

When checking that a module procedure definition is unique, allow for
the possibility that a submodule may contain a module procedure
interface that shadows a module procedure of the same name in its
(sub)module parent.   In other words, module procedure definitions
need only be unique in the tree of submodules rooted at the (sub)module
containing the relevant module procedure interface.

Differential Revision: https://reviews.llvm.org/D159033