[Clang] Pragma vectorize_width() implies vectorize(enable)

Let's try this again; this has been reverted/recommited a few times. Last time
this got reverted because for this loop:

  void a() {
    #pragma clang loop vectorize(disable)
    for (;;)
      ;
  }

vectorisation was incorrectly enabled and the vectorize.enable metadata was set
due to a logic error. But with this fixed, we now imply vectorisation when:

1) vectorisation is enabled, which means: VectorizeWidth > 1,
2) and don't want to add it when it is disabled or enabled, otherwise we would
   be incorrectly setting it or duplicating the metadata, respectively.

This should fix PR27643.

Differential Revision: https://reviews.llvm.org/D69628