lambdalifting: ignore AST literals for var capture

[saem]

Summary

nkNimNodeLit are no longer scanned during var capture detection,
ensuring that literals do not alter capture analysis of a routine
holding typed literal AST with symbols from the outer routine.

Details

Capture detection is done in two modules lambdalifting and sempass2
in the detectCapturedVars and detectCapture procedures,
respectively. With this change both procedures do not traverse into
nkNimNodeLit (treating them as more opaque literal data) for literal
analysis. A regression tests has been added a test for this (thanks to
Zerbina).

[zerbina]

Thanks. liftCapturedVars also needs to skip nkNimNodeLit nodes, but otherwise looks good to me.

As for a test case, the following snippet could be used:

proc sym() =
  discard

proc outer() {.compileTime.} =
  var x = 0

  proc sym() {.compileTime.} =
    # close over an outer local in order to make `sym` a closure procedure:
    x = 0

  proc inner() {.compileTime.} =
    let x = bindSym"sym" # create a symbol choice that includes the closure
                         # procedure

  # `inner` doesn't close over a local and also doesn't access a closure
  # procedure, so it must not be a closure procedure
  doAssert inner isnot "closure"

static:
  # call the procedure to make sure it passes the lambda-lifting pass
  outer()

Unfortunately, sempass2.detectCaptures also doesn't skip nkNimNodeLit nodes, so the test would fail even with the changes to lambdalifting.

[saem]

/merge

[github-actions]

Merge requested by: @saem

Contents after the first section break of the PR description has been removed and preserved below:

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Notes for Reviewers

• found during sym choices refactor: initial cut of the change #691
• it's an edge case, likely a non-issue after macro sanitization comes in