[Clang] Use DataLayout from TargetParser (#171135)
This switches clang to use the data layouts from TargetParser, instead
of maintaining its own copy of data layouts, which are required to match
the backend data layouts.
For now I've kept explicit calls to resetDataLayout(), just with the
argument implied by the triple and ABI. Ideally this would happen
automatically, but the way these classes are initialized currently
doesn't offer a great place to do this.
Previously resetDataLayout() also set the UserLabelPrefix. I've
separated this out, with a reasonable default so that most targets don't
need to worry about it.
I've kept the explicit data layouts for TCE and SPIR (without the V).
These seem to not correspond to real LLVM targets.
I've also fixed the XCore data layout in TargetParser, which was
incorrectly set to the same one as Xtensa. It was previously unused.
