Allow repeated registration again.

Our use of -fvisibility-inlines-hidden means we cannot check function pointers
against non null values.

Unfortunately, we also cannot assert that the callbacks are initialized only
once. The problem is that lldb has multiple subsystems that need to call this
and they don't have a unique initialization order.

Thanks to Sean Callanan for reporting it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192835 91177308-0d34-0410-b5e6-96231b3b80d8