[PM] Add a unittest for the CGSCC pass manager in the new pass manager
system.

Previously, this was only being tested with larger integration tests.
That makes it hard to isolated specific issues with it, and makes the
APIs themselves less well tested. Add a unittest based around the same
patterns used for testing the general pass manager.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@261624 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed