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

Re-land commit 3787fd942f

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

flang/lib/Parser/unparse.cpp

@@ -2307,17 +2307,7 @@ public:
   }
 
   void Unparse(const OmpAtomicDefaultMemOrderClause &x) {
-    switch (x.v) {
-    case OmpAtomicDefaultMemOrderClause::Type::SeqCst:
-      Word("SEQ_CST");
-      break;
-    case OmpAtomicDefaultMemOrderClause::Type::AcqRel:
-      Word("ACQ_REL");
-      break;
-    case OmpAtomicDefaultMemOrderClause::Type::Relaxed:
-      Word("RELAXED");
-      break;
-    }
+    Word(ToUpperCaseLetters(common::EnumToString(x.v)));
   }
 
   void Unparse(const OmpAtomicClauseList &x) { Walk(" ", x.v, " "); }