NewPM: Add an extension point for the start of the pipeline.

NewPM: Add an extension point for the start of the pipeline.

This applies to most pipelines except the LTO and ThinLTO backend
actions - it is for use at the beginning of the overall pipeline.

This extension point will be used to add the GCOV pass when enabled in
Clang.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323166 91177308-0d34-0410-b5e6-96231b3b80d8