Fix mingw build by moving the static const data member before the bitfields

Apparently GCC targeting Windows breaks bitfields on static data members:
  struct Foo {
    unsigned X : 16;
    static const int M = 42;
    unsigned Y : 16;
  };
  static_assert(sizeof(Foo) == 4, "asdf"); // fails

Who knew.

llvm-svn: 290700