This follows the design of how we handled this with
sil-verify-all. Specifically, the default behavior is to run only in asserts
builds, but one can use the two flags: enable-ast-verifier and
disable-ast-verifier to override the default behavior.
The reason why this is interesting is that this means that when compiling
normally, we will not run the verifier, so we won't have a perf hit. But we can
now ask the user to run with this flag (or in a future maybe a re-run in the
driver would do this for them), saving us time when screening bugs by avoiding
the need to build an asserts compiler to triage if the ASTVerifier would catch
the bug.
