[CodeGen] Extend Performance Counter to track per-scop information.
Previously, we would generate one performance counter for all scops.
Now, we generate both the old information, as well as a per-scop
performance counter to generate finer grained information.
This patch needed a way to generate a unique name for a Scop.
The start region, end region, and function name combined provides a
unique Scop name. So, Scop has a new public API to provide its start
and end region names.
Differential Revision: https://reviews.llvm.org/D33723