Rename ObjCMethodDecl::isSynthesized to isPropertyAccessor.

This more accurately reflects its use: this flag is set when a method
matches the getter or setter name for a property in the same class,
and does not actually specify whether or not the definition of the method
will be synthesized (either implicitly or explicitly with @synthesize).

This renames the setter and backing field as well, and changes the
(soon-to-be-obsolete?) XML dump format to use 'property_accessor'
instead of 'synthesized'.

llvm-svn: 165626