Fix nested namespace with decltype to hopefully work with MSVC

Build failed here:
http://lab.llvm.org:8011/builders/lld-x86_64-win7/builds/14629/steps/build_Lld/logs/stdio

So I'm taking a shot in the dark that MSVC (whatever version that is)
can't cope with nested name specifiers with a decltype prefix.

llvm-svn: 221931