[SemaExpr] Re-enable missing assertion

[davide]

Summary

This has been disabled for a long time, but:

1. Initializers work (and apparently they're re reason why this was disabled).
2. various tests happen to hit this code path and the invariant seems to be always verified.

I propose to add this back, it may be useful to catch mistakes.

[davide]

While at it, maybe the assertion can be modernized removing the != 0 part.

[rsmith]

LGTM

lib/Sema/SemaExpr.cpp

 
   return MaybeBindToTemporary(TheCall);
 }
 
 ExprResult
 Sema::ActOnCompoundLiteral(SourceLocation LParenLoc, ParsedType Ty,
                            SourceLocation RParenLoc, Expr *InitExpr) {
   assert(Ty && "ActOnCompoundLiteral(): missing type");
-  // FIXME: put back this assert when initializers are worked out.
-  //assert((InitExpr != 0) && "ActOnCompoundLiteral(): missing expression");
+  assert((InitExpr != 0) && "ActOnCompoundLiteral(): missing expression");
 
   TypeSourceInfo *TInfo;
   QualType literalType = GetTypeFromParser(Ty, &TInfo);
   if (!TInfo)
     TInfo = Context.getTrivialTypeSourceInfo(literalType);
 
   return BuildCompoundLiteralExpr(LParenLoc, TInfo, RParenLoc, InitExpr);
 }