[Debugify] Fix compile error in tracking coverage build

Forward-fixes a compile error in bc216b0 (#150212) in specific build
configurations, due to a missing const_cast.

Author: SLTozer

llvm/lib/Transforms/Utils/Debugify.cpp

@@ -712,7 +712,7 @@ bool llvm::checkDebugInfoMetadata(Module &M,
   // it is propagated to other instructions.
   for (auto &L : DILocsAfter)
     if (!L.second)
-      L.first->setDebugLoc(DebugLoc::getUnknown());
+      const_cast<Instruction *>(L.first)->setDebugLoc(DebugLoc::getUnknown());
 #endif
 
   bool ResultForVars = checkVars(DIVarsBefore, DIVarsAfter, NameOfWrappedPass,