[Flang][OpenMP][Sema] Support propagation of REQUIRES information across program units

Re-land commit 3787fd94

This patch adds support for storing OpenMP REQUIRES information in the
semantics symbols for programs/subprograms and modules/submodules, and
populates them during directive resolution. A pass is added to name resolution
that makes sure this information is also propagated across top-level programs,
functions and subprograms.

Storing REQUIRES information inside of semantics symbols will also allow
supporting the propagation of this information across Fortran modules. This
will come as a separate patch.

The `bool DirectiveAttributeVisitor::Pre(const parser::SpecificationPart &x)`
method is removed since it resulted in specification parts being visited twice.

This is patch 3/5 of a series splitting D149337 to simplify review.

Differential Revision: https://reviews.llvm.org/D157983