[Flang][OpenMP][Sema] More gracefully handle undefined symbol in a no implicit...

[Flang][OpenMP][Sema] More gracefully handle undefined symbol in a no implicit module for declare target

Prior to this change, if you define a module as such with a declare target in it:

module test_0
    implicit none
!$omp declare target(no_implicit_materialization_1)
end module test_0

The compiler will crash rather than give some form of reasonable
diagnostic. This patch attempts to fix that.

Reviewers: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D149913