Add a '-source' flag allowing compilation in e.g. 2.10 mode

This is aimed at letting the 2.11+ compiler operate, at least until
phase:typer, on a 2.10 codebase.
Puts SI-6899 fix under a source level flag
Put SI-7335 fix behind a source level flag
Adds YrelaxAliasVarianceChecks to possibly let SI-6566 slide by

Auto-activated by -source:2.10.