Hand over the job of laying out the argument structure
to the Materializer.  Materialization is still done by
the ClangExpressionDeclMap; this will be the next thing
to move.

Also fixed a layout bug that this uncovered.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@179318 91177308-0d34-0410-b5e6-96231b3b80d8