[gold-plugin] Avoid race condition when creating temporary files.

[gold-plugin] Avoid race condition when creating temporary files.

This is both a potential security issue and a potential functionality
issue because we create temporary files from multiple threads. Use
the safe version of createTemporaryFile instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310636 91177308-0d34-0410-b5e6-96231b3b80d8