Rename '_const' attribute to 'CompileTimeLiteral'

To pave the way for the new experimental feature which will operate on '@const' attribute and expand the scope of what's currently handled by '_const' without breaking compatibility, for now.

lib/Sema/CSDiagnostics.cpp

@@ -6474,7 +6474,7 @@ bool ExtraneousReturnFailure::diagnoseAsError() {
   return true;
 }
 
-bool NotCompileTimeConstFailure::diagnoseAsError() {
+bool NotCompileTimeLiteralFailure::diagnoseAsError() {
   emitDiagnostic(diag::expect_compile_time_const);
   return true;
 }