[AArch64][GlobalISel] Support lowering variadic musttail calls

This adds support for lowering variadic musttail calls. To do this, we have
to...

- Detect a musttail call in a variadic function before attempting to lower the
  call's formal arguments. This is done in the IRTranslator.
- Compute forwarded registers in `lowerFormalArguments`, and add copies for
  those registers.
- Restore the forwarded registers in `lowerTailCall`.

Because there doesn't seem to be any nice way to wrap these up into the outgoing
argument handler, the restore code in `lowerTailCall` is done separately.

Also, irritatingly, you have to make sure that the registers don't overlap with
any passed parameters. Otherwise, the scheduler doesn't know what to do with the
extra copies and asserts.

Add call-translator-variadic-musttail.ll to test this. This is pretty much the
same as the X86 musttail-varargs.ll test. We didn't have as nice of a test to
base this off of, but the idea is the same.

Differential Revision: https://reviews.llvm.org/D68043

llvm-svn: 373226