There are two main advantages to doing this
* Targets that only need to handle one of the formats specially don't have
to worry about the others. For example, x86 now only registers a
constructor for the COFF streamer.
* Changes to the arguments passed to one format constructor will not impact
the other formats.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232699 91177308-0d34-0410-b5e6-96231b3b80d8