Some cleanups to the native pdb plugin [NFC].

This is mostly some cleanup done in the process of implementing
some basic support for types.  I tried to split up the patch a
bit to get some of the NFC portion of the patch out into a separate
commit, and this is the result of that.  It moves some code around,
deletes some spurious namespace qualifications, removes some
unnecessary header includes, forward declarations, etc.

llvm-svn: 344913