Changed the recently added `FindConstantOnVariableDefinition` to
not rely on MemberAttributes being non-const.
Original commit message:
"""
This patch removes the Objective-C accessibility workaround added in
5a477cfd90
(rdar://8492646). This allows us to make the local `MemberAttributes`
variable immutable, which is useful for some other work around this
function I was planning on doing.
We don't need the workaround anymore since compiler-support for giving
debuggers access to private ivars was done couple of years later in
d6cb4a858d
(rdar://10997647).
**Testing**
* Test-suite runs cleanly
"""
f86770aa07