CodeGen: use COMDAT for block copy/destroy helpers

SVN r339438 added support to deduplicate the helpers by using a consistent
naming scheme and using LinkOnceODR semantics.  This works on ELF by means of
weak linking semantics, and entirely does not work on PE/COFF where you end up
with multiply defined strong symbols, which is a strong error on PE/COFF.
Assign the functions a COMDAT group so that they can be uniqued by the linker.
This fixes the use of blocks in CoreFoundation on Windows.

llvm-svn: 354678