Recommit [SchedBoundary] Add dump method for resource usage.

Summary:
As supporting information, I have added an example that describes how
the indexes of the vector of resources SchedBoundary::ReservedCycles
are tracked by the field SchedBoundary::ReservedCyclesIndex.

This has a minor rework of
https://github.com/llvm/llvm-project/commit/b39a9a94f420a25a239ae03097c255900cbd660e
which was reverted in
https://github.com/llvm/llvm-project/commit/df6ae1779fafd9984e144a27315d6dd65b32c325
becasue the llc invocation of the test was missing the argument
`-mtriple`.

See for example the failure at
https://lab.llvm.org/buildbot#builders/231/builds/7245 that reported
the following when targeting a non-aarch64 native build:

    'cortex-a55' is not a recognized processor for this target (ignoring processor)

Reviewers: jroelofs

Subscribers:

Differential Revision: https://reviews.llvm.org/D141367