TLS: Respect visibility for thread_local variables on Darwin (PR40327)

Summary:
Teach clang to mark thread wrappers for thread_local variables with
hidden visibility when the original variable is marked with hidden
visibility. This is necessary on Darwin which exposes the thread wrapper
instead of the thread variable. The thread wrapper would previously
always be created with default visibility unless it had
linkonce*/weak_odr linkage.

Reviewers: rjmccall

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D56818

llvm-svn: 351457