Make backtracking modifiers on alternations work correctly

[smls]

Summary

Current Rakudo behavior:

Backtrackable atom:	?	*	+	**	|	||
Respects default (eager backtracking) mode:	yes	yes	yes	yes	yes	yes
Respects ratchet mode:	yes	yes	yes	yes	yes	no
Respects atom-specific backtracking modifier:	yes	yes	yes	yes	yes	no
Atom-specific backmod overrides ratchet mode:	yes	yes	yes	yes	no	n/a

With this pull request, the behavior becomes "yes" for all 24 cases.

Implementation

Please review carefully, because these are my first commits to nqp and my understanding of this code base is limited... 😃
I basically just copied the existing no-backtrack handling code from method alt to method altseq in both the MoarVM and JVM backend (263257a), and fixed a thinko in the quantified_atom action method where the "backtrack" flag for alternation atoms is set in the first place (ba165c8).

RT & SYN

This fixes:

• RT #130117
• RT #131973

It also matches the behavior intended for these backtracking modifiers by S05, as I understand it.

Roast

This PR passed make spectest on my machine (Linux/MoarVM) .

One it's merged, I'd suggest adding the following additional tests to roast:

use Test;
plan 24;

is "ab" ~~ / a .? b /,            "ab", "backtrack into ?";
is "ab" ~~ / a .?: b /,           Nil,  "don't backtrack into ?:";
is "ab" ~~ / :r a .? b /,         Nil,  "don't backtrack into ? after :r";
is "ab" ~~ / :r a .?! b /,        "ab", "backtrack into ?! (overrides :r)";

is "ab" ~~ / a .* b /,            "ab", "backtrack into *";
is "ab" ~~ / a .*: b /,           Nil,  "don't backtrack into *:";
is "ab" ~~ / :r a .* b /,         Nil,  "don't backtrack into * after :r";
is "ab" ~~ / :r a .*! b /,        "ab", "backtrack into *! (overrides :r)";

is "ab" ~~ / .+ b /,              "ab", "backtrack into +";
is "ab" ~~ / .+: b /,             Nil,  "don't backtrack into +:";
is "ab" ~~ / :r .+ b /,           Nil,  "don't backtrack into + after :r";
is "ab" ~~ / :r .+! b /,          "ab", "backtrack into +! (overrides :r)";

is "ab" ~~ / . ** 1..3 b /,       "ab", "backtrack into **";
is "ab" ~~ / . **: 1..3 b /,      Nil,  "don't backtrack into **:";
is "ab" ~~ / :r . ** 1..3 b /,    Nil,  "don't backtrack into ** after :r";
is "ab" ~~ / :r . **! 1..3 b /,   "ab", "backtrack into **! (overrides :r)";

is "ab" ~~ / [ab | a ] b /,       "ab", "backtrack into |";
is "ab" ~~ / [ab | a ]: b /,      Nil,  "don't backtrack into | with :";
is "ab" ~~ / :r [ab | a ] b /,    Nil,  "don't backtrack into | after :r";
is "ab" ~~ / :r [ab | a ]:! b /,  "ab", "backtrack into | with :! (overrides :r)";

is "ab" ~~ / [ab || a ] b /,      "ab", "backtrack into ||";
is "ab" ~~ / [ab || a ]: b /,     Nil,  "don't backtrack into || with :";
is "ab" ~~ / :r [ab || a ] b /,   Nil,  "don't backtrack into || after :r";
is "ab" ~~ / :r [ab || a ]:! b /, "ab", "backtrack into || with :! (overrides :r)";

Of these tests, the following three fail without this PR:

not ok 20 - backtrack into | with :! (overrides :r)
not ok 22 - don't backtrack into || with :
not ok 23 - don't backtrack into || after :r

With this PR applied, all 24 tests succeed.

[jnthn]

Seems reasonable to me; thanks!