[GWP-ASan] Use functions in backtrace test, not line numbers.

Summary:
There's no unwinding functionality on Android that allows for line
numbers to be retrieved in-process. As a result, we can't have
this backtrace test run on Android.

Cleanup the test to use optnone functions instead, which is more stable
than line numbers anyway.

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: #sanitizers, morehouse, cferris

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D76807