[llvm] [llvm-objcopy][COFF] Update WinCFGuard section contents after stripping (PR #153322)

[Evgenii Kudriashov via llvm-commits]

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@@ -92,6 +93,63 @@ Error COFFWriter::finalizeSymbolContents() {
   return Error::success();
 }
 
+Error COFFWriter::finalizeCFGuardContents() {
+  DenseMap<size_t, size_t> SymIdMap;
+  bool NeedUpdate = false;
+  for (Symbol &Sym : Obj.getMutableSymbols()) {
+    NeedUpdate |= Sym.OriginalRawIndex == Sym.RawIndex;
+    SymIdMap[Sym.OriginalRawIndex] = Sym.RawIndex;
+  }
+
+  if (!NeedUpdate)
+    return Error::success();
+
+  for (auto &Sym : Obj.getMutableSymbols()) {
+    if (Sym.Name != ".gljmp$y" && Sym.Name != ".giats$y" &&
+        Sym.Name != ".gfids$y")
+      continue;
+
+    auto Sec = find_if(Obj.getMutableSections(),
+                       [&Sym](Section &S) { return S.Name == Sym.Name; });
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e-kud wrote:

It was my first version, then I found that I can use `findSymbol` using a section. But then I realized it returns `const`. So, this is how I came up with the backward version. What if we build a `StringMap` from Symbol name to `coff_aux_section_definition` while collecting id mapping (basically three entries at max). Then we can iterate through sections and we know where `Checksum` should be written.

https://github.com/llvm/llvm-project/pull/153322