libstdc++: Check lengths first in operator== for basic_string [PR62187]

libstdc++: Check lengths first in operator== for basic_string [PR62187]

As confirmed by LWG 2852, the calls to traits_type::compare do not need
to be obsvervable, so we can make operator== compare string lengths
first and return immediately for non-equal lengths. This avoids doing a
slow string comparison for "abc...xyz" == "abc...xy". Previously we only
did this optimization for std::char_traits<char>, but we can enable it
unconditionally thanks to LWG 2852.

For comparisons with a const char* we can call traits_type::length right
away to do the same optimization. That strlen call can be folded away
for constant arguments, making it very efficient.

For the pre-C++20 operator== and operator!= overloads we can swap the
order of the arguments to take advantage of the operator== improvements.

libstdc++-v3/ChangeLog:

	PR libstdc++/62187
	* include/bits/basic_string.h (operator==): Always compare
	lengths before checking string contents.
	[!__cpp_lib_three_way_comparison] (operator==, operator!=):
	Reorder arguments.