63e00ce106
This PR ports more of the implementation of availability checking from
clang-rt into the Engine. In particular, when the call to look up the
symbol `_availability_version_check` fails, this PR falls back on
reading the platform version information out of a plist file at a
well-known location, as is done
[here](2fd66e6eb6/compiler-rt/lib/builtins/os_version_check.c (L163)).
This change fixes a mistake in
https://github.com/flutter/engine/pull/44711, which didn't account for
`_availability_version_check` not being available on iOS 11 and 12.
Fixes https://github.com/flutter/flutter/issues/138711