<regex>: Improve search performance for regexes with initial + quantifiers

[muellerj2]

Towards #5468. This is a small change that greatly speeds up searches for regexes like a+ that start with some letter/string/character class followed by a + quantifier (or any other quantifier requiring at least one repetition). Because this loop must be matched at least once, we can enter the repeated subpattern and look for the first position a letter/string/character class in the subpattern can match.

While working on this, I noticed that I didn't think the implementation of text_regex::should_search_match_capture_groups() through well enough: I designed it to use relative coordinates for expected submatches, but this isn't so helpful when one wants to ensure that the whole match is in a particular position. So I changed the implementation to use absolute coordinates from the start of the matched string. Luckily, no test seems to have relied on the previous behavior, meaning all of them just matched the start of the input string anyway.

Benchmark

Running on my machine:

Benchmark	Before	After	Speedup
bm_lorem_search/"bibe"/2	38504 ns	39237 ns	0.98
bm_lorem_search/"bibe"/3	76730 ns	76730 ns	1.00
bm_lorem_search/"bibe"/4	153460 ns	153460 ns	1.00
bm_lorem_search/"(bibe)+"/2	4814680 ns	92076 ns	52.29
bm_lorem_search/"(bibe)+"/3	9521484 ns	204041 ns	46.66
bm_lorem_search/"(bibe)+"/4	18158784 ns	401088 ns	45.27
bm_lorem_search/"(?:bibe)+"/2	4743304 ns	97656 ns	48.57
bm_lorem_search/"(?:bibe)+"/3	9521484 ns	192540 ns	49.45
bm_lorem_search/"(?:bibe)+"/4	19531250 ns	384976 ns	50.73

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

I resolved a trivial adjacent-add conflict with #5494 in VSO_0000000_regex_use.

[StephanTLavavej]

➕ 🚀 ⏱️