`<regex>`: `regex_search()` throws `regex_error(error_complexity)` with an unwisely written pattern

Related to #6347.

This was reported by the compiler back-end team, reduced from an important test case. This was previously accepted (MSVC Build Tools 14.50 and earlier) with regex_search() returning false (the line has been deliberately changed to contain total_WOOF_pushes so total_heap_pushes won't be found). Now with MSVC Build Tools 14.51 and later, it is rejected with a complexity exception.

D:\GitHub\STL\out\x64>type regex_repro.cpp

#include <print>
#include <regex>
#include <string>
using namespace std;

int main() {
    const regex rx(R"(.*total_heap_pushes\s*:\s*(\d+) .*)");

    const string line = "Router Stats: total_nets_routed: 113175 "
                        "total_connections_routed: 437370 "
                        "total_WOOF_pushes: 313639277 "
                        "total_heap_pops: 35866777 "
                        "total_internal_heap_pushes: 0 "
                        "total_internal_heap_pops: 0 "
                        "total_external_heap_pushes: 313639277 "
                        "total_external_heap_pops: 35866777 "
                        "total_external_SOURCE_pushes: 379297 "
                        "total_external_SOURCE_pops: 379297 "
                        "total_internal_SOURCE_pushes: 0 "
                        "total_internal_SOURCE_pops: 0 "
                        "rt_node_SOURCE_pushes: 379297 "
                        "rt_node_SOURCE_high_fanout_pushes: 3797 "
                        "rt_node_SOURCE_entire_tree_pushes: 375500 "
                        "total_external_SINK_pushes: 3488243 "
                        "total_external_SINK_pops: 3260194 "
                        "total_internal_SINK_pushes: 0 "
                        "total_internal_SINK_pops: 0 "
                        "rt_node_SINK_pushes: 0 "
                        "rt_node_SINK_high_fanout_pushes: 0 "
                        "rt_node_SINK_entire_tree_pushes: 0 "
                        "total_external_IPIN_pushes: 5501600 "
                        "total_external_IPIN_pops: 3585046 "
                        "total_internal_IPIN_pushes: 0 "
                        "total_internal_IPIN_pops: 0 "
                        "rt_node_IPIN_pushes: 0 "
                        "rt_node_IPIN_high_fanout_pushes: 0 "
                        "rt_node_IPIN_entire_tree_pushes: 0 "
                        "total_external_OPIN_pushes: 14718566 "
                        "total_external_OPIN_pops: 8542877 "
                        "total_internal_OPIN_pushes: 0 "
                        "total_internal_OPIN_pops: 0 "
                        "rt_node_OPIN_pushes: 342094 "
                        "rt_node_OPIN_high_fanout_pushes: 8165 "
                        "rt_node_OPIN_entire_tree_pushes: 333929 "
                        "total_external_CHANX_pushes: 143852606 "
                        "total_external_CHANX_pops: 10396956 "
                        "total_internal_CHANX_pushes: 0 "
                        "total_internal_CHANX_pops: 0 "
                        "rt_node_CHANX_pushes: 2213020 "
                        "rt_node_CHANX_high_fanout_pushes: 482509 "
                        "rt_node_CHANX_entire_tree_pushes: 1730511 "
                        "total_external_CHANY_pushes: 145698965 "
                        "total_external_CHANY_pops: 9702407 "
                        "total_internal_CHANY_pushes: 0 "
                        "total_internal_CHANY_pops: 0 "
                        "rt_node_CHANY_pushes: 2442282 "
                        "rt_node_CHANY_high_fanout_pushes: 589528 "
                        "rt_node_CHANY_entire_tree_pushes: 1852754 "
                        "total_number_of_adding_all_rt: 8814821 "
                        "total_number_of_adding_high_fanout_rt: 62167 "
                        "total_number_of_adding_all_rt_from_calling_high_fanout_rt: 41981 ";

    try {
        const bool found = regex_search(line, rx);
        println("regex_search() returned {}.", found);
    } catch (const regex_error& re) {
        println("{}", re.what());
    }
}

D:\GitHub\STL\out\x64>cl /EHsc /nologo /W4 /std:c++latest /MTd /Od regex_repro.cpp
regex_repro.cpp

D:\GitHub\STL\out\x64>regex_repro
regex_error(error_complexity): The complexity of an attempted match against a regular expression exceeded a pre-set level.

In this case, telling the user to fix their regex is not especially feasible. (The initial ".*" used with a regex_search() is bad and should feel bad.) I would like to retune the limits to accept this case, while still guarding against severely pathological regex DoS scenarios.

Start by running the provided regex_repro.cpp case and trace the complexity handling behind regex_search() and regex_error(error_complexity). Identify the existing limits and related regression coverage; done means this case returns false without throwing while pathological regular expressions remain protected from excessive work.