[Clang][DebugInfo] Cease turning instruction-referencing off by default

Over in D114631 I turned this debug-info feature on by default, for x86_64
only. I'd previously stripped out the clang cc1 option that controlled it
in 651122fc, unfortunately that turned out to not be completely
effective, and the two things deleted in this patch continued to keep it
off-by-default.  Oooff.

As a follow-up, this patch removes the last few things to do with
ValueTrackingVariableLocations from clang, which was the original purpose
of D114631. In an ideal world, if this patch causes you trouble you'd
revert 3c045070 instead, which was where this behaviour was supposed
to start being the default, although that might not be practical any more.