Expose lookup for feature-availability decl on ASTContext.

Swift needs to be able to look up the `VarDecl` that defines a given feature
availability domain.

Author: tshortli

clang/include/clang/AST/ASTContext.h

@@ -840,6 +840,15 @@ class ASTContext : public RefCountedBase<ASTContext> {
 
   bool hasFeatureAvailabilityAttr(const Decl *D) const;
 
+  /// Retrieve the \c VarDecl that represents the feature availability domain
+  /// with the given name.
+  VarDecl *getFeatureAvailDecl(StringRef Name) const {
+    auto It = AvailabilityDomainMap.find(Name);
+    if (It != AvailabilityDomainMap.end())
+      return It->second;
+    return nullptr;
+  }
+
   // Retrieve availability domain information for a feature.
   AvailabilityDomainInfo getFeatureAvailInfo(StringRef FeatureName) const;