[Compile Time Values] Add syntactic verification of valid expressions in '@const' contexts

Syntactically verify that initializer expressions of '@const' variables and argument expressions to '@const' parameters consist strictly of syntactically-verifiable set of basic values and operations

lib/AST/FeatureSet.cpp

@@ -408,6 +408,10 @@ static bool usesFeatureCompileTimeValues(Decl *decl) {
          decl->getAttrs().hasAttribute<ConstInitializedAttr>();
 }
 
+static bool usesFeatureCompileTimeValuesPreview(Decl *decl) {
+  return false;
+}
+
 static bool usesFeatureClosureBodyMacro(Decl *decl) {
   return false;
 }