[AST] Visit memtransfer arguments in order

The only point to this change is the test diffs.  When I remove this code entirely (in favor of the recently added generic handling), I don't want there to be any confusion due to spurious test diffs.

As an aside, the fact out tests are AST construction order dependent is not great.  I thought about fixing that, but the reasonable schemes I might want (e.g. sort by name) need the test diffs anyways.

Philip

llvm-svn: 341841