Issue 1341: Elevate statevars to enclosing block when wrapped

[jstuder-gh]

Previously in some blocks following the "do" statement prefix, statevars
would be seemingly reset after each iteration of the loop.

Given the statement:

say do loop { state $x = 0; last if ++$x > 2; $x }

This statement would result in multiple thunk refs being produced, and
ultimately, would be wrapped in a block that is used as a cloning target
in looping and a new container would be declared and initialized for
each iteration.

The QAST would look like this (trimmed to relevant parts):

- QAST::Block(:cuid(3) :blocktype(declaration_static))
  - QAST::Stmts  { state $x = 0; last if ++$x > 2; $x }
  - QAST::Stmts
    - QAST::Block(:cuid(1) :blocktype(immediate)) <wanted>
      - QAST::Stmts
        - QAST::Var(lexical $x :decl(statevar))
        - QAST::Var(lexical $_ :decl(var))

After this commit the QAST looks like this:

- QAST::Block(:cuid(3) :blocktype(declaration_static))
  - QAST::Stmts  { state $x = 0; last if ++$x > 2; $x }
    - QAST::Var(lexical $x :decl(statevar))
  - QAST::Stmts
    - QAST::Block(:cuid(1) :blocktype(immediate)) <wanted>
      - QAST::Stmts
        - QAST::Op(null)
        - QAST::Var(lexical $_ :decl(var))

This way the statevar is declared in the block being cloned and thus the
same container is used in each iteration.

Note the enclosing block is a block newly created via the make_thunk_ref sub,
and contains no child nodes other than the first Stmts (that has no children prior
to this commit) and the second Stmts containing the cuid(1) subtree.

See Github Issue 1341

[zoffixznet]

👍 Thanks!