[DWARF] Unify warning callbacks. NFC.

Both DWARFDebugLine and DWARFDebugAddr used the same callback mechanism
for handling recoverable errors. They both implemented similar warn() function
to be used as such callbacks.

In this revision we get rid of code duplication and move this warn() function
to DWARFContext as DWARFContext::dumpWarning().

Reviewers: lhames, jhenderson, aprantl, probinson, dblaikie, JDevlieghere

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D51033

llvm-svn: 340528