[Darwin][Driver] Don't pass a file as object_path_lto during ThinLTO

Summary:
After r327851, Driver::GetTemporaryPath will create the file rather than
just create a potientially unqine filename. If clang driver pass the
file as parameter as -object_path_lto, ld64 will pass it back to libLTO
as GeneratedObjectsDirectory, which is going to cause a LLVM ERROR if it
is not a directory.
Now during thinLTO, pass a temp directory path to linker instread.

rdar://problem/47194182

Reviewers: arphaman, dexonsmith

Reviewed By: arphaman

Subscribers: mehdi_amini, inglorion, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D56608

llvm-svn: 350970
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index 977bff5..4186fe2 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -4478,6 +4478,17 @@
   return Path.str();
 }
 
+std::string Driver::GetTemporaryDirectory(StringRef Prefix) const {
+  SmallString<128> Path;
+  std::error_code EC = llvm::sys::fs::createUniqueDirectory(Prefix, Path);
+  if (EC) {
+    Diag(clang::diag::err_unable_to_make_temp) << EC.message();
+    return "";
+  }
+
+  return Path.str();
+}
+
 std::string Driver::GetClPchPath(Compilation &C, StringRef BaseName) const {
   SmallString<128> Output;
   if (Arg *FpArg = C.getArgs().getLastArg(options::OPT__SLASH_Fp)) {