[COFF] Keep the underscore on exported decorated stdcall functions

[mstorsjo]

Summary

This (together with the corresponding LLD commit) fixes PR35733.

[mstorsjo]

The matching change to the update of D41632.

[mstorsjo]

@ruiu Is this associated patch ok as well?

[mstorsjo]

Ping @ruiu @rnk - the corresponding change for LLD was already OK'd by Rui in D41632, but these two need to go together.

[ruiu]

LGTM

include/llvm/Object/COFFImportFile.h

struct COFFShortExport {
   std::string ExtName;
   std::string SymbolName;
 
   uint16_t Ordinal = 0;
   bool Noname = false;
   bool Data = false;
   bool Private = false;
   bool Constant = false;
+  bool KeepDecoration = false;
 
   bool isWeak() {
     return ExtName.size() && ExtName != Name;
   }
 
   friend bool operator==(const COFFShortExport &L, const COFFShortExport &R) {
     return L.Name == R.Name && L.ExtName == R.ExtName &&
             L.Ordinal == R.Ordinal && L.Noname == R.Noname &&

lib/Object/COFFImportFile.cpp

for (COFFShortExport E : Exports) {
     ImportType ImportType = IMPORT_CODE;
     if (E.Data)
       ImportType = IMPORT_DATA;
     if (E.Constant)
       ImportType = IMPORT_CONST;
 
     StringRef SymbolName = E.SymbolName.empty() ? E.Name : E.SymbolName;
     ImportNameType NameType = getNameType(SymbolName, E.Name, Machine);
+    if (E.KeepDecoration)
+      NameType = IMPORT_NAME;
     Expected<std::string> Name = E.ExtName.empty()
                                      ? SymbolName
                                      : replace(SymbolName, E.Name, E.ExtName);
 
     if (!Name)
       return Name.takeError();
 
     Members.push_back(

lib/Object/COFFModuleDefinition.cpp

if (Tok.K == Equal) {
       E.Name = Tok.Value;
     } else {
       unget();
     }
 
     if (Machine == IMAGE_FILE_MACHINE_I386) {
       if (!isDecorated(E.Name, MingwDef))
         E.Name = (std::string("_").append(E.Name));
+      else if (!MingwDef)
+        E.KeepDecoration = true;
       if (!E.ExtName.empty() && !isDecorated(E.ExtName, MingwDef))
         E.ExtName = (std::string("_").append(E.ExtName));
     }
 
     for (;;) {
       read();
       if (Tok.K == Identifier && Tok.Value[0] == '@') {
         if (Tok.Value == "@") {