[LTO] Driver support for -lto_library flag

This commit adds support for the -lto_library flag, allowing users to specify a custom LTO library on Darwin. This also fixes an issue where the default LTO library is used even if Driver is run from inside an alternate toolchain.

lib/Driver/Driver.cpp

@@ -1448,6 +1448,12 @@ void Driver::buildOutputInfo(const ToolChain &TC, const DerivedArgList &Args,
       Diags.diagnose(SourceLoc(), diag::error_invalid_arg_value,
                      A->getAsString(Args), A->getValue());
   }
+  
+  if (const Arg *A = Args.getLastArg(options::OPT_lto_library)) {
+    OI.LibLTOPath = A->getValue();
+  } else {
+    OI.LibLTOPath = "";
+  }
 
   auto CompilerOutputType = OI.LTOVariant != OutputInfo::LTOKind::None
                              ? file_types::TY_LLVM_BC