Introduce DebugInfoProbe. This is used to monitor how llvm optimizer is...

Introduce DebugInfoProbe. This is used to monitor how llvm optimizer is treating debugging information. 
It generates output that lools like

8 times line number info lost by Scalar Replacement of Aggregates (SSAUp) 
1 times line number info lost by Simplify well-known library calls 
12 times variable info lost by Jump Threading

llvm-svn: 127381