Debug Info: move StripDebugInfo from StripSymbols.cpp to DebugInfo.cpp.

We can share the implementation between StripSymbols and dropping debug info
for metadata versions that do not match.

Also update the comments to match the implementation. A follow-on patch will
drop the "Debug Info Version" module flag in StripDebugInfo.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195505 91177308-0d34-0410-b5e6-96231b3b80d8