[OpenCL] Fix diagnostic of thread_local

[OpenCL] Fix diagnostic of thread_local

Commit 08c258670c ("[OpenCL] Generic address space has been added in
OpenCL v2.0.", 2014-11-26) did not mark the thread_local keyword as a
storage class specifier, whereas it did mark _Thread_local as such.
According to the C++14 spec s7.1.1, thread_local is a storage class
specifier, so mark it as such.

I will add a test for this in a follow-up commit that adds keyword
restrictions to the OpenCL C++ language mode.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@330721 91177308-0d34-0410-b5e6-96231b3b80d8