Fixes an issue where a nil-coalescing operation used in a boolean
context would result in no control flow out of the default operand of
the nil-coalescing operator.
This fixes `IfConfigDecl` for both the AST printer and control flow
libraries.
It turns out that the active lements of an `IfConfigDecl` are already
listed in the enclosing scope (like a `BraceStmt`), so they should not
be listed as children, and `IfConfigDecl` can be jsut a leaf in the
control flow.
This introduces a `MethodRefExpr` node synthesized out of
`DotSyntaxCallExpr` under the `LookupExpr` hierarchy. This means that
much like
```free_function(1, 2)```
is a `CallExpr` with `getFunction` giving a `DeclRefExpr`,
```foo.method(1, 2)```
is now a `CallExpr` with `getFunction` giving a `MethodRefExpr`.
`ApplyExpr::getStaticTarget` has been made work with it (as well as
`ConstructorRefCallExpr` which for the moment has been left where it
is), a new `MethodApplyExpr` has been introduced deriving from it,
and control and data flow libraries have adapted.
A small but was fixed in `qlgen` where the default constructor for DB
types was not correctly subtracting derived IPA types depending on the
order of definitions in `schema.yml`.
There are still some occurrences of `DotSyntaxCallExpr`, and as already
mentioned the other `SelfApply` class (`ConstructorRefCallExpr`) was
left alone. Their treatment is left for a future PR.
