[DWARFLinker] Remove unused declaration keepDIEAndDependencies

The corresponding function definition was removed by:

  commit 95a8e8a
  Author: Jonas Devlieghere <jonas@devlieghere.com>
  Date:   Tue Dec 3 11:10:04 2019 -0800

llvm/include/llvm/DWARFLinker/DWARFLinker.h

@@ -563,13 +563,6 @@ class DWARFLinker {
                         OffsetsStringPool &DebugLineStrPool,
                         unsigned Indent = 0);
 
-  /// Mark the passed DIE as well as all the ones it depends on as kept.
-  void keepDIEAndDependencies(AddressesMap &RelocMgr, RangesTy &Ranges,
-                              const UnitListTy &Units, const DWARFDie &DIE,
-                              CompileUnit::DIEInfo &MyInfo,
-                              const DWARFFile &File, CompileUnit &CU,
-                              bool UseODR);
-
   unsigned shouldKeepDIE(AddressesMap &RelocMgr, RangesTy &Ranges,
                          const DWARFDie &DIE, const DWARFFile &File,
                          CompileUnit &Unit, CompileUnit::DIEInfo &MyInfo,

llvm/lib/DWARFLinker/DWARFLinker.cpp

@@ -903,8 +903,6 @@ void DWARFLinker::lookForDIEsToKeep(AddressesMap &AddressesMap,
     if ((Current.Flags & TF_DependencyWalk) && AlreadyKept)
       continue;
 
-    // We must not call shouldKeepDIE while called from keepDIEAndDependencies,
-    // because it would screw up the relocation finding logic.
     if (!(Current.Flags & TF_DependencyWalk))
       Current.Flags = shouldKeepDIE(AddressesMap, Ranges, Current.Die, File,
                                     Current.CU, MyInfo, Current.Flags);