[pdb] Re-add code to write PDB files.

Somehow all the functionality to write PDB files got removed,
probably accidentally when uploading the patch perhaps the wrong
one got uploaded.  This re-adds all the code, as well as the
corresponding test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274248 91177308-0d34-0410-b5e6-96231b3b80d8
12 files changed