[ELF][LTO] Fix non-v2 LTO build

If a given linker LTO plugin does not support the v3 API, we resolve
all symbol names to identify the set of object files that are actually
needed for linking and then restart the linker to do the same thing
again from scratch, excluding the unneeded files from the beginning.

We do this because the v2 API does not provide a way to "unload" an
object file once it is read.

To identify an unneeded object file on the second run, we serialize
its name in the form of `<archive-name>:<offset-in-archive>` or just
a filename. If an object file is in a thin archive, we simply used
its filename.

But if the same file is directly given to a linker as well as as a
thin archive member, the file would accidentally be excluded on the
second run.

This change fixes that issue. With this change, it looks like we can
build GCC 12 with LTO using mold.

Fixes #454

archive-file.h

@@ -93,6 +93,7 @@ read_thin_archive_members(C &ctx, MappedFile<C> *mf) {
     std::string path = name.starts_with('/') ?
       name : (filepath(mf->name).parent_path() / name).string();
     vec.push_back(MappedFile<C>::must_open(ctx, path));
+    vec.back()->thin_parent = mf;
     data = body;
   }
   return vec;

elf/main.cc

@@ -45,6 +45,7 @@ static ObjectFile<E> *new_lto_obj(Context<E> &ctx, MappedFile<Context<E>> *mf,
 
   ObjectFile<E> *file = read_lto_object(ctx, mf);
   file->priority = ctx.file_priority++;
+  file->archive_name = archive_name;
   file->is_in_lib = ctx.in_lib || (!archive_name.empty() && !ctx.whole_archive);
   file->is_alive = !file->is_in_lib;
   ctx.has_lto_object = true;

mold.h

@@ -590,6 +590,12 @@ class MappedFile {
       // because archive members may have the same name.
       return parent->name + ":" + std::to_string(get_offset());
     }
+
+    if (thin_parent) {
+      // If this is a thin archive member, the filename part is
+      // guaranteed to be unique.
+      return thin_parent->name + ":" + name;
+    }
     return name;
   }
 
@@ -599,6 +605,7 @@ class MappedFile {
   i64 mtime = 0;
   bool given_fullpath = true;
   MappedFile *parent = nullptr;
+  MappedFile *thin_parent = nullptr;
   int fd = -1;
 };