[Driver] Limit .exe extension addition to Windows hosts

[smeenai]

Summary

The driver should invoke lld as lld-link rather than lld-link.exe
when cross compiling.

The alternative would be to teach lld's build system to always produce
lld-link.exe even on non-Windows hosts, but I think it's cleaner to
change the driver to respect the host system's executable suffix (or
lack thereof).

[smeenai]

Forgot to fix a test

[compnerd]

Why not always replace the extension? Windows doesnt require the .exe suffix IIRC.

[smeenai]

Why not always replace the extension? Windows doesnt require the .exe suffix IIRC.

CreateProcess documentation says "This parameter must include the file name extension; no default extension is assumed.". That's kinda surprising to me, since LoadLibrary assumes a .dll extension.

[smeenai]

Will just drop the .exe entirely, since that works

lib/Driver/ToolChains/MSVC.cpp

if (TC.getIsVS2017OrNewer() &&
         }
         Cursor += EnvVar.size() + 1 /*null-terminator*/;
       }
     }
   SkipSettingEnvironment:;
 #endif
   } else {
     linkPath = Linker;
+    if (llvm::Triple(LLVM_HOST_TRIPLE).isOSWindows())
       llvm::sys::path::replace_extension(linkPath, "exe");
     linkPath = TC.GetProgramPath(linkPath.c_str());
   }
 
   auto LinkCmd = llvm::make_unique<Command>(
       JA, *this, Args.MakeArgString(linkPath), CmdArgs, Inputs);
   if (!Environment.empty())
     LinkCmd->setEnvironment(Environment);
   C.addCommand(std::move(LinkCmd));