[flang] Fix checking of TBP bindings

Non-DEFERRED procedure binding checking can't blindly accept
all procedures defined in modules -- they can't be ABSTRACT
interfaces.  And GetUltimate() must be used rather than
FindSubprogram(); the latter will resolve to a procedure's
interface in the case of "procedure(interface) :: external",
not "external".

Differential Revision: https://reviews.llvm.org/D145749