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
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index 4033e65..ac4eea4 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -218,6 +218,9 @@
   }
 }
 
+// Define out of line so we don't have to include DwarfUnit.h in DwarfDebug.h.
+DwarfDebug::~DwarfDebug() { }
+
 // Switch to the specified MCSection and emit an assembler
 // temporary label to it if SymbolStem is specified.
 static MCSymbol *emitSectionSym(AsmPrinter *Asm, const MCSection *Section,