MCContext already has many tasks, and separating CodeView out from it is
probably a good idea. The .cv_loc tracking was modelled on the DWARF
tracking which lived directly in MCContext.
Removes the inclusion of MCCodeView.h from MCContext.h, so now there are
only 10 build actions while I hack on CodeView support instead of 265.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279847 91177308-0d34-0410-b5e6-96231b3b80d8