[SourceKit] Add a new query to collect all group names from a module.

Input:
  Request Id: 'source.request.module.groups'
  Required field: 'key.modulename'

Output:
  Array of group names in that module, if any.

include/swift/Serialization/SerializedModuleLoader.h

@@ -140,6 +140,8 @@ public:
 
   Optional<StringRef> getGroupNameForDecl(const Decl *D) const override;
 
+  void collectAllGroups(std::vector<StringRef> &Names) const override;
+
   virtual void getTopLevelDecls(SmallVectorImpl<Decl*> &results) const override;
 
   virtual void