Vectorize basic_string::rfind (the string needle overload)

[AlexGuteniev]

Towards #5036.

Similar to #5048, the differences:

• The parameters calculations that use _Start_at are trickier
• The input_needle.empty() special case in the tests is really special case this time
• Used intentionally not-very-aligned strings, towards Provide an intentional (mis)alignment that corresponds to typical usage in benchmarks for plain arrays #5035

Benchmark	main	this
member_rfind<not_highly_aligned_string>/0	108 ns	23.2 ns
member_rfind<not_highly_aligned_string>/1	101 ns	22.6 ns
member_rfind<not_highly_aligned_string>/2	1298 ns	305 ns
member_rfind<not_highly_aligned_string>/3	1554 ns	347 ns
member_rfind<not_highly_aligned_string>/4	7914 ns	4870 ns
member_rfind<not_highly_aligned_string>/5	15104 ns	15482 ns
member_rfind<not_highly_aligned_wstring>/0	98.1 ns	39.7 ns
member_rfind<not_highly_aligned_wstring>/1	101 ns	38.9 ns
member_rfind<not_highly_aligned_wstring>/2	1288 ns	624 ns
member_rfind<not_highly_aligned_wstring>/3	1559 ns	722 ns
member_rfind<not_highly_aligned_wstring>/4	5645 ns	9777 ns
member_rfind<not_highly_aligned_wstring>/5	15808 ns	20634 ns

STL/benchmarks/src/search.cpp

Lines 41 to 49 in a1bc126

	constexpr data_and_pattern patterns[] = {
	/* 0. Small, closer to end */ {lorem_ipsum, "aliquet"sv},
	/* 1. Large, closer to end */ {lorem_ipsum, "aliquet malesuada"sv},
	/* 2. Small, closer to begin */ {lorem_ipsum, "pulvinar"sv},
	/* 3. Large, closer to begin */ {lorem_ipsum, "dapibus elit interdum"sv},
	/* 4. Small, evil */ {fill_pattern_view<3000, false>, fill_pattern_view<7, true>},
	/* 5. Large, evil */ {fill_pattern_view<3000, false>, fill_pattern_view<20, true>},
	};