[lldb] Don't emit a warning when using Objective-C getters in expressions

Clang emits a warning when accessing an Objective-C getter but not using the result.
This gets triggered when just trying to print a getter value in the expression parser (where
Clang just sees a normal expression like `obj.getter` while parsing).

This patch just disables the warning in the expression parser (similar to what we do with
the C++ equivalent of just accessing a member variable but not doing anything with it).

Reviewed By: kastiglione

Differential Revision: https://reviews.llvm.org/D94307