[ExprConstant] Try fixing __builtin_constant_p after D54355 (rC347417)

Summary:
Reinstate the original behavior (Success(false, E)) before D54355 when this branch is
taken. This fixes spurious error of the following snippet:

  extern char extern_var;
  struct { int a; } a = {__builtin_constant_p(extern_var)};

llvm-svn: 348037

Author: MaskRay

clang/lib/AST/ExprConstant.cpp

@@ -8199,7 +8199,6 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
       // We can delay calculation of __builtin_constant_p until after
       // inlining. Note: This diagnostic won't be shown to the user.
       Info.FFDiag(E, diag::note_invalid_subexpr_in_const_expr);
-      return false;
     }
     return Success(false, E);
   }