Do assignment lvalue error checking in type checking

Added some tests related to this.
Also improved source file position reporting in error reporting.

expr.cpp

@@ -2037,12 +2037,8 @@ AssignExpr::GetValue(FunctionEmitContext *ctx) const {
     ctx->SetDebugPos(pos);
 
     Symbol *baseSym = lvalue->GetBaseSymbol();
-    if (!baseSym) {
-        // FIXME: I think that this check also is unnecessary and that this
-        // case should be covered during type checking.
-        Error(pos, "Left hand side of assignment statement can't be assigned to.");
-        return NULL;
-    }
+    // Should be caught during type-checking...
+    assert(baseSym != NULL);
 
     switch (op) {
     case Assign: {
@@ -2180,6 +2176,12 @@ AssignExpr::TypeCheck() {
         }
     }
 
+    if (lvalue->GetBaseSymbol() == NULL) {
+        Error(lvalue->pos, "Left hand side of assignment statement can't be "
+              "assigned to.");
+        return NULL;
+    }
+
     const Type *lhsType = lvalue->GetType();
     if (dynamic_cast<const PointerType *>(lhsType) != NULL) {
         if (op == AddAssign || op == SubAssign) {