swift234

Forward base rvalues directly to accessors instead of
conservatively copying them.

Also, fix a number of issues with mutating getters that
I noticed while examining and changing this code.  In
particular, stop relying on suppressing writeback scopes
during loads.

Fixes rdar://19002913, a bug where an unnecessary copy of
an array for a getter call left the array in a non-unique
state when a subsequent mutation occurred.

Swift SVN r23642