Change the `linker_private_weak_def_auto' linkage to `linkonce_odr_auto_hide' to

Change the `linker_private_weak_def_auto' linkage to `linkonce_odr_auto_hide' to
make it more consistent with its intended semantics.

The `linker_private_weak_def_auto' linkage type was meant to automatically hide
globals which never had their addresses taken. It has nothing to do with the
`linker_private' linkage type, which outputs the symbols with a `l' (ell) prefix
among other things.

The intended semantic is more like the `linkonce_odr' linkage type.

Change the name of the linkage type to `linkonce_odr_auto_hide'. And therefore
changing the semantics so that it produces the correct output for the linker.

Note: The old linkage name `linker_private_weak_def_auto' will still parse but
is not a synonym for `linkonce_odr_auto_hide'. This should be removed in 4.0.
<rdar://problem/11754934>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162114 91177308-0d34-0410-b5e6-96231b3b80d8