Add backend name to Target to enable runtime info to be fed back into TableGen

Add backend name to Target to enable runtime info to be fed back into TableGen

Summary:
Make it possible to feed runtime information back to tablegen to enable
profile-guided tablegen-eration, detection of untested tablegen definitions, etc.

Being a cross-compiler by nature, LLVM will potentially collect data for multiple
architectures (e.g. when running 'ninja check'). We therefore need a way for
TableGen to figure out what data applies to the backend it is generating at the
time. This patch achieves that by including the name of the 'def X : Target ...'
for the backend in the TargetRegistry.

Reviewers: qcolombet

Reviewed By: qcolombet

Subscribers: jholewinski, arsenm, jyknight, aditya_nandakumar, sdardis, nemanjai, ab, nhaehnle, t.p.northover, javed.absar, qcolombet, llvm-commits, fedor.sergeev

Differential Revision: https://reviews.llvm.org/D39742

llvm-svn: 318352