[lldb][test] Remove failing ValueCheck on deprecated libcxx entity

A recent libcxx change renamed all internal variables starting with
`__`. As such, `std::reverse_iterator::__t` was renamed to
`std::reverse_iterator::__t_`. This breaks the `import-std-module`
tests with newer libcxx versions. Since this variable is deprecated
in libcxx anyway, this patch simply removes the explicit `ValueCheck`
on the variable name. We don't lose any interesting test-case here
since the purpose of the test is to see if we can call functions
from the `std` module.

We can now re-enable the tests on Darwin for all buildbot Clang
compiler variants.

Differential Revision: https://reviews.llvm.org/D134727