Wdeprecated: RegionBindingsRef are copy constructed, make sure that's safe by...

Wdeprecated: RegionBindingsRef are copy constructed, make sure that's safe by removing the unnecessary user-declared copy assignment operator

The user-defined copy assignment looks like it was working around the
presence of a reference member (that probably doesn't change in the copy
assignment cases present in the program). Rather than continuing this - just
change the reference to a pointer and let all the special members be
defined implicitly.

llvm-svn: 244974