Make DWARFCompileUnit non-copyable

Summary:
This is a part of D1164. DWARFCompileUnit is not that lightweight
to copy it around, and we want it to own corresponding .dwo compile unit
eventually.

Reviewers: echristo

Reviewed By: echristo

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1298