[msf] Create LLVMDebugInfoMsf

This provides a better layering of responsibilities among different
aspects of PDB writing code.  Some of the MSF related code was
contained in CodeView, and some was in PDB prior to this.  Further,
we were often saying PDB when we meant MSF, and the two are
actually independent of each other since in theory you can have
other types of data besides PDB data in an MSF.  So, this patch
separates the MSF specific code into its own library, with no
dependencies on anything else, and DebugInfoCodeView and
DebugInfoPDB take dependencies on DebugInfoMsf.

llvm-svn: 276458