[MCA] Moved View.h and View.cpp from /tools/llvm-mca/ to /lib/MCA/.

Moved View.h and View.cpp from /tools/llvm-mca/Views/ to /lib/MCA/ and
/include/llvm/MCA/. This is so that targets can define their own Views within
the /lib/Target/ directory (so that the View can use backend functionality).
To enable these Views within mca, targets will need to add them to the vector of
Views returned by their target's CustomBehaviour::getViews() methods.

Differential Revision: https://reviews.llvm.org/D108520