Identify object files compiled with cl.exe /GL.

[ruiu]

Summary

Object files compiled with cl.exe /GL contain intermediate code for LTO.
We can't (and don't want to) interpret such code, but we should print
out a user-friendly error message.

[pcc]

LGTM

lld/trunk/COFF/Driver.cpp

Driver->Cpio->append(relativeToRoot(MB.getBufferIdentifier()),
                          MB.getBuffer());
 
   // File type is detected by contents, not by file extension.
   file_magic Magic = identify_magic(MB.getBuffer());
   if (Magic == file_magic::archive)
     return std::unique_ptr<InputFile>(new ArchiveFile(MB));
   if (Magic == file_magic::bitcode)
     return std::unique_ptr<InputFile>(new BitcodeFile(MB));
+  if (Magic == file_magic::coff_cl_gl_object)
+    fatal(MB.getBufferIdentifier() + ": is not a native COFF file. "
+          "Recompile without /GL");
   if (Config->OutputFile == "")
     Config->OutputFile = getOutputPath(MB.getBufferIdentifier());
   return std::unique_ptr<InputFile>(new ObjectFile(MB));
 }
 
 static bool isDecorated(StringRef Sym) {
   return Sym.startswith("_") || Sym.startswith("@") || Sym.startswith("?");
 }

lld/trunk/test/COFF/cl-gl.test

+# RUN: not lld-link /out:%t.exe /entry:main %S/Inputs/cl-gl.obj >& %t.log
+# RUN: FileCheck %s < %t.log
+
+# CHECK: is not a native COFF file. Recompile without /GL