[llvm][DWARFLinker] Fix gcc 13 -Wuninitialized warnings (#143867)

A bit awkward that we have to switch from public to protected and back
again, but it seemed neater than putting OS all the way down at the
bottom. Since it is a public member that you're more likely to be
looking for.

llvm-project/llvm/lib/DWARFLinker/Parallel/OutputSections.h:157:67:
warning: member
‘llvm::dwarf_linker::parallel::SectionDescriptor::Contents’ is used
uninitialized [-Wuninitialized]

Which refers to the use in the constructor:
```
  SectionDescriptor(DebugSectionKind SectionKind, LinkingGlobalData &GlobalData,
                    dwarf::FormParams Format, llvm::endianness Endianess)
      : SectionDescriptorBase(SectionKind, Format, Endianess), OS(Contents),
```
Where Contents is passed to `OS`, before Contents has been constructed.

Author: DavidSpickett

llvm/lib/DWARFLinker/Parallel/OutputSections.h

@@ -181,6 +181,11 @@ struct SectionDescriptor : SectionDescriptorBase {
   /// to the debug section, corresponding to this object.
   uint64_t StartOffset = 0;
 
+protected:
+  /// Section data bits.
+  OutSectionDataTy Contents;
+
+public:
   /// Stream which stores data to the Contents.
   raw_svector_ostream OS;
 
@@ -287,9 +292,6 @@ struct SectionDescriptor : SectionDescriptorBase {
 
   LinkingGlobalData &GlobalData;
 
-  /// Section data bits.
-  OutSectionDataTy Contents;
-
   /// Some sections are generated using AsmPrinter. The real section data
   /// located inside elf file in that case. Following fields points to the
   /// real section content inside elf file.