[libcxx] Use alias for detecting overriden function (#120805)
This mechanism is preferable in environments like embedded since it
doesn't require special handling of the custom section.
This is a reland of https://github.com/llvm/llvm-project/pull/114961
which addresses the issue reported by downstream users. Specifically,
the two differences from the previous version are:
* The internal `symbol##_impl__` symbol in the Mach-O implementation is
annotated with `__attribute__((used))` to prevent LTO from deleting it
which we've seen in the previous version.
* `__is_function_overridden` is marked as `inline` so these symbols are
placed in a COMDAT (or fully inlined) to avoid duplicate symbol errors
which we've seen in the previous version.
llvm-project
84189554 - [libcxx] Use alias for detecting overriden function (#120805)
