[TLS on Darwin] change how we handle globals with linkonce or weak linkage.

[TLS on Darwin] change how we handle globals with linkonce or weak linkage.

This is about how we handle static member of a template. Before this commit,
we use internal linkage for the IR thread-local variable, which is inefficient.
With this commit, we will start to follow Itanium C++ ABI.

rdar://problem/23415206

Reviewed by John McCall.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@252814 91177308-0d34-0410-b5e6-96231b3b80d8