Do not access beyond the end of local symbols.

[ruiu]

Summary

This patch resurrects code that was removed in r317007 to not access
beyond allocated memory for a symbol.

[pcc]

LGTM

lld/COFF/SymbolTable.cpp

if (!Undef)
       continue;
     if (!Sym->IsUsedInRegularObj)
       continue;
 
     StringRef Name = Undef->getName();
 
     // A weak alias may have been resolved, so check for that.
     if (Defined *D = Undef->getWeakAlias()) {
+      // We want to replace Sym with D. However, we can't just blindly
+      // copy sizeof(SymbolUnion) bytes from D to Sym because D may be an
+      // internal symbol, and internal symbols are stored as "unparented"
+      // Symbols. For that reason we need to check which type of symbol we
+      // are dealing with and copy the correct number of bytes.
+      if (isa<DefinedRegular>(D))
+        memcpy(Sym, D, sizeof(DefinedRegular));
+      else if (isa<DefinedAbsolute>(D))
+        memcpy(Sym, D, sizeof(DefinedAbsolute));
+      else
         memcpy(Sym, D, sizeof(SymbolUnion));
       continue;
     }
 
     // If we can resolve a symbol by removing __imp_ prefix, do that.
     // This odd rule is for compatibility with MSVC linker.
     if (Name.startswith("__imp_")) {
       SymbolBody *Imp = find(Name.substr(strlen("__imp_")));
       if (Imp && isa<Defined>(Imp)) {