[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