[CodeGen][Darwin] Set the calling-convention of thread-local variable

initialization functions to 'cxx_fast_tlscc'.

This fixes a bug where instructions calling initialization functions for
thread-local static members of c++ template classes were using calling
convention 'cxx_fast_tlscc' while the called functions weren't annotated
with the calling convention.

rdar://problem/40447463

Differential Revision: https://reviews.llvm.org/D47354

llvm-svn: 333447