Fix missing diagnostic for unsupported TLS for some thread_local variables.

Clang r181627 moved a check for block-scope variables into this code for
handling thread storage class specifiers, but in the process, it broke the
logic for checking if the target supports TLS. Fix this with some simple
restructuring of the code. rdar://problem/18796883

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222512 91177308-0d34-0410-b5e6-96231b3b80d8