[Flang][OpenMP] Push context when parsing DECLARE VARIANT

Basic parsing and semantics support for Declare Variant was added in
PR 130578. However, this did not include variant of `Pre` and `Post`
within `OmpAttributeVisitor`. This meant that when a function in the
class tried to get the context using `GetContext`, Flang would crash
as the context was empty. To ensure this is possible, such as when
resolving names as part of the `uniform` clause in the `simd` directive,
the context is now pushed within `OmpAttributeVisitor` when parsing a
`DECLARE VARIANT` directive.

Fixes #145222

Author: Stylie777

flang/lib/Semantics/resolve-directives.cpp

@@ -495,6 +495,12 @@ class OmpAttributeVisitor : DirectiveAttributeVisitor<llvm::omp::Directive> {
   bool Pre(const parser::OpenMPAllocatorsConstruct &);
   void Post(const parser::OpenMPAllocatorsConstruct &);
 
+  bool Pre(const parser::OmpDeclareVariantDirective &x) {
+    PushContext(x.source, llvm::omp::Directive::OMPD_declare_variant);
+    return true;
+  }
+  void Post (const parser::OmpDeclareVariantDirective &) { PopContext(); };
+
   void Post(const parser::OmpObjectList &x) {
     // The objects from OMP clauses should have already been resolved,
     // except common blocks (the ResolveNamesVisitor does not visit

flang/test/Parser/OpenMP/declare-variant.f90

@@ -102,3 +102,30 @@ subroutine vsub(v1, v2)
     type(c_ptr), value :: v1, v2
   end
 end subroutine
+
+subroutine f
+  real :: x, y
+  y = 2
+  !omp simd
+  call f2(x, y)
+  !omp end simd 
+contains
+  subroutine f1 (x, y)
+    real :: x, y
+  end
+
+  subroutine f2 (x, y)
+    real :: x, y
+    !$omp declare variant (f1) match (construct={simd(uniform(y))})
+  end
+end subroutine
+!CHECK: !$OMP DECLARE VARIANT (f1) MATCH(CONSTRUCT={SIMD(UNIFORM(y))})
+!PARSE-TREE: | | | | DeclarationConstruct -> SpecificationConstruct -> OpenMPDeclarativeConstruct -> OmpDeclareVariantDirective
+!PARSE-TREE-NEXT: | | | | | Verbatim
+!PARSE-TREE-NEXT: | | | | | Name = 'f1'
+!PARSE-TREE-NEXT: | | | | | OmpClauseList -> OmpClause -> Match -> OmpMatchClause -> OmpContextSelectorSpecification -> OmpTraitSetSelector
+!PARSE-TREE-NEXT: | | | | | | OmpTraitSetSelectorName -> Value = Construct
+!PARSE-TREE-NEXT: | | | | | | OmpTraitSelector
+!PARSE-TREE-NEXT: | | | | | | | OmpTraitSelectorName -> Value = Simd
+!PARSE-TREE-NEXT: | | | | | | | Properties
+!PARSE-TREE-NEXT: | | | | | | | | OmpTraitProperty -> OmpClause -> Uniform -> Name = 'y'