Rust: Remove nonsentical no-match CFG edges

[paldepind]

Cleans up the handling of patterns that can't fail and makes it more precise such that no-match edges that will never be taken are removed.

The added tests also exposed a bug for macro patterns. I haven't fixed that as it's orthogonal to this PR.

Here's an example. Previously we had a no-match edge out of MyStruct {...} but that is now gone as the struct pattern itself will never cause a match failure:

flowchart TD
1["enter fn struct_pattern"]
10["MyStruct {...}"]
11["1"]
11["1"]
13["0"]
14["MyStruct {...}"]
15["x"]
15["x"]
17["3"]
2["exit fn struct_pattern"]
3["exit fn struct_pattern (normal)"]
4["st"]
4["st"]
6["...: MyStruct"]
7["{ ... }"]
8["match st { ... }"]
9["st"]

1 --> 4
3 --> 2
4 -- match --> 6
4 --> 4
6 --> 9
7 --> 3
8 --> 7
9 --> 10
10 -- match --> 11
11 -- match --> 13
11 -- no-match --> 14
11 --> 11
13 --> 8
14 -- match --> 15
15 -- match --> 17
15 --> 15
17 --> 8

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[hvitved]

Thanks a lot for doing this. Some small suggestions.

[hvitved]

Suggested change

	* If this pattern is immediately nested within another pattern, then get the
	* parent pattern.
	* Gets the pattern under which this pattern is immediately nested, if any.

[hvitved]

Should we reintroduce the and not p = any(Variable v).getPat() constraint? I realize that the variable logic makes its own assumptions about what might be a variable binding or path pattern though...

[hvitved]

RecordPat also needs to be added; enum variants are allowed to use record fields.

[hvitved]

This QL doc seems to suggest the dual of what the predicate actually computes? I wonder if it is actually better to implement what the QL doc says, e.g.

private predicate cannotCauseMatchFailure(Pat pat) {
  pat = any(IdentPat p | p.hasPat())
  or
  pat instanceof BoxPat
  or
  pat instanceof RestPat
  or
  pat instanceof MacroPat
  or
  pat instanceof WildcardPat
  or
  pat instanceof RangePat
  or
  pat instanceof RefPat
  or
  pat instanceof SlicePat
  or
  pat instanceof TuplePat
  or
  pat instanceof ConstBlockPat
}

and then replace not canCauseMatchFailure(pat) with cannotCauseMatchFailure(pat) further down.