[lldb] Fix import-std-module tests after libc++ got a new __memory subdirectory

7ad49aec added a __memory subdirectory to libc++
but the code we use to find libc++ from the debug info support files wasn't
prepared to encounter unknown subdirectories within libc++. The import-std-module
tests automatically fell back to not importing the std module which caused
them to fail.

This patch removes our hardcoded exception for the 'experimental' subdirectory
and instead just ignores all subdirectories of c++/vX/ when searching the
support files.