Introduce lowering phase that turns !!exp -> exp for the boolean
'not' builtin. This makes sure that code such as
```
if (!!!!!booleanValue) {
doStuff()
}
```
generates only one branch.
Most of these tests used this directive as a way to opt in to a new
language feature, and most of those features are already stable for a
long time, so no opt-in is needed. Some other tests used the directive
to opt out from a language feature, replace those by the `LANGUAGE`
directive. One test used the directive to test behavior that actually
depended on the API version; use `API_VERSION` directive there instead.
- Turn some const conditions into non-const conditions
- Make sure inlined const values are used where required
(otherwise they are eliminated by POP backward propagation)
