[RFC][Patch 2/3] Add a MCSubtargetInfo hook to resolve variant scheduling classes.

This patch is the second of a sequence of three patches related to LLVM-dev RFC
"MC support for varinat scheduling classes".
https://lists.llvm.org/pipermail/llvm-dev/2018-May/123181.html

The goal of this patch is to enable the resolution of variant classes in MC with
the help of a new method named `MCSubtargetInfo::resolveVariantSchedClass()`.

This patch also teaches the SubtargetEmitter how to automatically generate the
definition of method resolveVariantSchedClass().  That definition is emitted
within a sub-class of MCSubtargetInfo named XXXGenMCSubtargetInfo (where XXX is
the name of the Target).

Differential Revision: https://reviews.llvm.org/D47077

llvm-svn: 333286