[MCA] Introduce class LSUnitBase and let LSUnit derive from it.

Class LSUnitBase provides a abstract interface for all the concrete LS units in
llvm-mca.

Methods exposed by the public abstract LSUnitBase interface are:
 - Status isAvailable(const InstRef&);
 - void dispatch(const InstRef &);
 - const InstRef &isReady(const InstRef &);

LSUnitBase standardises the API, but not the data structures internally used by
LS units. This allows for more flexibility.
Previously, only method `isReady()` was declared virtual by class LSUnit.
Also, derived classes had to inherit all the internal data members of LSUnit.

No functional change intended.

llvm-svn: 361496