Fixes a possible assert in the custom SubsetSubject logic for the attr emitter.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195962 91177308-0d34-0410-b5e6-96231b3b80d8
Using a custom subject to reenable the Subjects line for the ns_bridged attribute. No functional change intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195963 91177308-0d34-0410-b5e6-96231b3b80d8
Refactored the tls_model attribute to use a custom subset subject. No functional change intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195964 91177308-0d34-0410-b5e6-96231b3b80d8