[flang] Check procedure pointer initializations; clean up ELEMENTAL

Implements compatibility checking for initializers in procedure pointer
declarations.  This work exposed some inconsistency in how ELEMENTAL
interfaces were handled and checked, from both unrestricted intrinsic
functions and others, and some refinements needed for function result
compatbility checking; these have also been ironed out.  Some new
warnings are now emitted, and this affected a dozen or so tests.

Differential Revision: https://reviews.llvm.org/D159026