[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

llvm-svn: 304528