Fix the clang-cl self-host build by defining ~DwarfDebug out of line

DwarfDebug.h has a SmallVector member containing a unique_ptr of an
incomplete type.  MSVC doesn't have key functions, so the vtable and
dtor are emitted in AsmPrinter.cpp, where DwarfDebug's ctor is called.
AsmPrinter.cpp include DwarfUnit.h and doesn't get a complete definition
of DwarfTypeUnit.  We could fix the problem by including DwarfUnit.h in
DwarfDebug.h, but that would increase header bloat.  Instead, define
~DwarfDebug out of line.

llvm-svn: 207701