[libc++][CI] Fix check-generated-output.

Before the script detected non-ASCII characters but let them pass. This
fixes the issue. I had a way to solve the issue, during review @Quuxplusone
suggested a better alternative. The patch has been changed to use this alternative.

Intended failed builds:
- Not updated generated files https://buildkite.com/llvm-project/libcxx-ci/builds/2822
- Not updated generated files and non-ASCII usage https://buildkite.com/llvm-project/libcxx-ci/builds/2835
- Non-ASCII usage https://buildkite.com/llvm-project/libcxx-ci/builds/2836

Reviewed By: #libc, Quuxplusone, curdeius

Differential Revision: https://reviews.llvm.org/D101303