Revert "[Clang] CWG1473: do not err on the lack of space after operator"""

This reverts commit f2583f3a.

There is a large body of non-conforming C-like code using format strings
like this:

  #define PRIuS "zu"
  void h(size_t foo, size_t bar) {
    printf("foo is %"PRIuS", bar is %"PRIuS, foo, bar);
  }

Rejecting this code would be very disruptive. We could decide to do
that, but it's sufficiently disruptive that I think it requires
gathering more community consensus with an RFC, and Aaron indicated [1]
it's OK to revert for now so continuous testing systems can see past
this issue while we decide what to do.

[1] https://reviews.llvm.org/D153156#4607717