[MCA] Return a mask of busy resources from method ResourceManager::checkAvailability(). NFCI

In case of bottlenecks caused by pipeline pressure, we want to be able to
correctly report the set of problematic pipelines. This is a first step towards
adding support for bottleneck hints in llvm-mca (see PR37494). No functional
change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353706 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed