Sink dwarf unit length emission down into DwarfUnit::emitHeader

This allows the CU label to be emitted only for compile units, as
they're the only ones that need it (so they can be referenced from
pubnames)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221072 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/AsmPrinter/DwarfCompileUnit.h

@@ -177,6 +177,8 @@ public:
     return sizeof(uint32_t) + // Length field
         getHeaderSize() + UnitDie.getSize();
   }
+
+  void emitHeader(const MCSymbol *ASectionSym) const override;
 };
 
 } // end llvm namespace