Parser: add explicit body discriminators

[zhenrongliew]

fixed the parser to be:
CFG:

    fn @f(...) -> T cfg {
      ^entry(...) {
        ...
      }
    }

Standalone Block body:

    fn @f(...) -> T block ^body(...) {
      ...
    }

DiGraph:

    fn @f(...) -> T digraph ^g0(...) {
      ...
    }

UnGraph:

    fn @f(...) -> T ungraph ^u0(...) {
      ...
    }

A CFG’s constituent blocks remain untagged:

    ... cfg {
      ^entry { ... }
      ^next { ... }
    }
    ```