Fix LLVMgold plugin name/path for non-Linux.

[danalbert]

Summary

It's only named LLVMgold.so on Linux. Fix the name for Windows and
Darwin.

Also fix the path for Windows so binutils doesn't have to.

[srhines]

Looks great. Thanks for fixing this up. It's hard to believe that there is nothing in Support that gives you the proper suffix for shared libraries. It would seem like this might be useful elsewhere, but I actually didn't find really any other place that wants to use this value generically.

[danalbert]

In D35739#817705, @srhines wrote:

It's hard to believe that there is nothing in Support that gives you the proper suffix for shared libraries. It would seem like this might be useful elsewhere, but I actually didn't find really any other place that wants to use this value generically.

I didn't see anything, but these terms show up often enough that it was difficult to grep.

cfe/trunk/lib/Driver/ToolChains/CommonArgs.cpp

 
 void tools::AddGoldPlugin(const ToolChain &ToolChain, const ArgList &Args,
                           ArgStringList &CmdArgs, bool IsThinLTO,
                           const Driver &D) {
   // Tell the linker to load the plugin. This has to come before AddLinkerInputs
   // as gold requires -plugin to come before any -plugin-opt that -Wl might
   // forward.
   CmdArgs.push_back("-plugin");
-  std::string Plugin =
-      ToolChain.getDriver().Dir + "/../lib" CLANG_LIBDIR_SUFFIX "/LLVMgold.so";
+
+#if defined(LLVM_ON_WIN32)
+  const char *Suffix = ".dll";
+#elif defined(__APPLE__)
+  const char *Suffix = ".dylib";
+#else
+  const char *Suffix = ".so";
+#endif
+
+  SmallString<1024> Plugin;
+  llvm::sys::path::native(Twine(ToolChain.getDriver().Dir) +
+                              "/../lib" CLANG_LIBDIR_SUFFIX "/LLVMgold" +
+                              Suffix,
+                          Plugin);
   CmdArgs.push_back(Args.MakeArgString(Plugin));
 
   // Try to pass driver level flags relevant to LTO code generation down to
   // the plugin.
 
   // Handle flags for selecting CPU variants.
   std::string CPU = getCPUName(Args, ToolChain.getTriple());
   if (!CPU.empty())