CodeGen: Don't emit a thread-wrapper if we can't touch the backing variable

OS X TLS has all accesses going through the thread-wrapper function and
gives the backing thread-local variable internal linkage.  This means
that thread-wrappers must have WeakAnyLinkage so that references to the
internal thread-local variables do not get propagated to other code.

It also means that translation units which do not provide a definition
for the thread-local variable cannot attempt to emit a thread-wrapper
because the thread wrapper will attempt to reference the backing
variable.

Differential Revision: http://reviews.llvm.org/D4109