[LLD] Zero-initialization & explicit set-up of isUsedinRegObject in Symbol class

It happened due to lld's COFF linker multiple regression tests failure.
It got reliably reproduced after the needed intialization of
isUsedinRegularObject bit in the Symbol's ctor, but not handled at
replaceSymbol API properly while creating a specific symbol to insert in
symbol table.

Eventually, the issue was about not handling exported symbols properly,
when dealing with defining undefined symbols for COFF linker. It got
resolved by explicit setting up the isUsedinRegularObject bit around the
ctor invocation of symbol class in replaceSymbol API.

Author: vg0204

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.");
   }
@@ -499,8 +499,10 @@ void replaceSymbol(Symbol *s, ArgT &&... arg) {
   assert(static_cast<Symbol *>(static_cast<T *>(nullptr)) == nullptr &&
          "Not a Symbol");
   bool canInline = s->canInline;
+  bool isUsedInRegularObj = s->isUsedInRegularObj;
   new (s) T(std::forward<ArgT>(arg)...);
   s->canInline = canInline;
+  s->isUsedInRegularObj = isUsedInRegularObj;
 }
 } // namespace coff