[libc++][CI] fixed a return curr_symbol() for Russian in the...

[libc++][CI] fixed a return curr_symbol() for Russian in the libcxx/test/support/locale_helpers.h for AIX

Summary:

fixed a return curr_symbol() for Russian in the libcxx/test/support/locale_helpers.h for AIX

Reviewers:  David Tenty,Mark de Wever
Differential Revision: https://reviews.llvm.org/D125801