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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189089 91177308-0d34-0410-b5e6-96231b3b80d8