Fix #4938 - Be stricter wrt. supported AddrExp in toConstElem()

DMD seems to support AddrExp of struct literals only...

Author: kinke

gen/toconstelem.cpp

@@ -401,7 +401,7 @@ class ToConstElemVisitor : public Visitor {
       return;
     }
 
-    // address of indexExp
+    /* address of indexExp
     if (auto iexp = e->e1->isIndexExp()) {
       // indexee must be global static array var
       VarExp *vexp = iexp->e1->isVarExp();
@@ -426,6 +426,7 @@ class ToConstElemVisitor : public Visitor {
       result = gep;
       return;
     }
+    */
 
     if (auto se = e->e1->isStructLiteralExp()) {
       result = p->getStructLiteralGlobal(se);
@@ -449,12 +450,7 @@ class ToConstElemVisitor : public Visitor {
       return;
     }
 
-    if (e->e1->op == EXP::slice || e->e1->op == EXP::dotVariable) {
-      visit(static_cast<Expression *>(e));
-      return;
-    }
-
-    llvm_unreachable("unsupported AddrExp in ToConstElemVisitor");
+    visit(static_cast<Expression *>(e));
   }
 
   //////////////////////////////////////////////////////////////////////////////

tests/fail_compilation/gh4938.d

@@ -0,0 +1,4 @@
+// RUN: not %ldc -c %s 2>&1 | FileCheck %s
+
+// CHECK: gh4938.d(4): Error: expression `&"whoops"w[0]` is not a constant
+immutable(wchar)* x = &"whoops"w[0];