Unimplemented toConstElem is not a fatal error if errors are gagged.

Because ldc uses gagging to check whether an expression is constant.

gen/toir.cpp

@@ -3022,7 +3022,8 @@ STUB(PowAssignExp);
 
 #define CONSTSTUB(x) LLConstant* x::toConstElem(IRState * p) { \
     error("expression '%s' is not a constant", toChars()); \
-    fatal(); \
+    if (!global.gag) \
+        fatal(); \
     return NULL; \
 }
 CONSTSTUB(Expression);