[COFF] Initialize, save, and restore isUsedByRegularObj

Before this change, SymbolTable::insert set this boolean to a meaningful
value before calling the appropriate Symbol constructor via
replaceSymbol. I believe this is UB, it is probably valid for a compiler
to zero out a bitfield in the constructor, rather than carefully
preserving the single bit that existed prevoiusly.

After this change, the constructor does the obvious thing, which is to
zero initialize this field, and we explicitly copy the bitfield around
the constructor call in replaceSymbol. This should be an alternative
solution to the problems described in llvm#98447 .

Author: rnk

lld/COFF/Symbols.h

@@ -98,10 +98,10 @@ class Symbol {
   friend SymbolTable;
   explicit Symbol(Kind k, StringRef n = "")
       : symbolKind(k), isExternal(true), isCOMDAT(false),
-        writtenToSymtab(false), pendingArchiveLoad(false), isGCRoot(false),
-        isRuntimePseudoReloc(false), deferUndefined(false), canInline(true),
-        isWeak(false), nameSize(n.size()),
-        nameData(n.empty() ? nullptr : n.data()) {
+        writtenToSymtab(false), isUsedInRegularObj(false),
+        pendingArchiveLoad(false), isGCRoot(false), isRuntimePseudoReloc(false),
+        deferUndefined(false), canInline(true), isWeak(false),
+        nameSize(n.size()), nameData(n.empty() ? nullptr : n.data()) {
     assert((!n.empty() || k <= LastDefinedCOFFKind) &&
            "If the name is empty, the Symbol must be a DefinedCOFF.");
   }
@@ -498,9 +498,13 @@ void replaceSymbol(Symbol *s, ArgT &&... arg) {
                 "SymbolUnion not aligned enough");
   assert(static_cast<Symbol *>(static_cast<T *>(nullptr)) == nullptr &&
          "Not a Symbol");
+  // These booleans are set to meaningful values during symbol table insertion.
+  // Save and restore them after constructing the Symbol subclass.
   bool canInline = s->canInline;
+  bool isUsedInRegularObj = s->isUsedInRegularObj;
   new (s) T(std::forward<ArgT>(arg)...);
   s->canInline = canInline;
+  s->isUsedInRegularObj = isUsedInRegularObj;
 }
 } // namespace coff