[TargetSchedule] Expose sub-units of a ProcResGroup in MCProcResourceDesc.

Summary:
Right now using a ProcResource automatically counts as usage of all
super ProcResGroups. All this is done during codegen, so there is no
way for schedulers to get this information at runtime.

This adds the information of which individual ProcRes units are
contained in a ProcResGroup in MCProcResourceDesc.

Reviewers: gchatelet

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D43023

llvm-svn: 324582