Add getOperatorDecls to FileUnit and ModuleDecl

Query the SourceLookupCache for the operator decls,
and use ModuleDecl::getOperatorDecls for both
frontend stats and to clean up some code
completion logic.

In addition, this commit switches getPrecedenceGroups
over to querying SourceLookupCache.

lib/Serialization/SerializedModuleLoader.cpp

@@ -1184,6 +1184,11 @@ void SerializedASTFile::getTopLevelDeclsWhereAttributesMatch(
   File.getTopLevelDecls(results, matchAttributes);
 }
 
+void SerializedASTFile::getOperatorDecls(
+    SmallVectorImpl<OperatorDecl *> &results) const {
+  File.getOperatorDecls(results);
+}
+
 void SerializedASTFile::getPrecedenceGroups(
        SmallVectorImpl<PrecedenceGroupDecl*> &results) const {
   File.getPrecedenceGroups(results);