`tests.py` should map `/std:c++latest` to `'c++2b'`

[StephanTLavavej]

tests.py maps compiler options to "features", used by the libcxx test suite:

STL/tests/utils/stl/test/tests.py

Lines 234 to 241 in 4c862ee

	if flag[5:] == 'c++latest':
	self._addCustomFeature('c++2a')
	elif flag[5:] == 'c++20':
	self.requires.append('unsupported_potato') # TRANSITION, VS2019 Preview 16.11
	elif flag[5:] == 'c++17':
	self._addCustomFeature('c++17')
	elif flag[5:] == 'c++14':
	self._addCustomFeature('c++14')

The toolset update #1953 is mapping /std:c++20 to 'c++2a'. However, it's still mapping /std:c++latest to 'c++2a'. This prevents libcxx's C++23 tests from running.

For example, #1950 is implementing is_scoped_enum, but attempting to unskip its test

STL/tests/libcxx/skipped_tests.txt

Lines 278 to 279 in 4c862ee

	# C++23 P1048R1 "is_scoped_enum"
	utilities\meta\meta.unary\meta.unary.prop\is_scoped_enum.pass.cpp

leads to it being detected as unsupported. (The test marks itself as unsupported in C++03 through C++2a inclusive, meaning it expects C++2b.)

As I recall, I initially attempted to map /std:c++latest to 'c++2b' in toolset update #1953, but encountered a couple of libcxx tests that weren't expecting this. This might be resolved if we update to libcxx's latest commit.

[SuperWig]

This might be resolved if we update to libcxx's latest commit.

I can confirm that works. I updated the commit and got

Unexpectedly Passed Tests (2):
libc++ :: std/utilities/meta/meta.unary/meta.unary.prop/is_scoped_enum.pass.cpp:0
libc++ :: std/utilities/meta/meta.unary/meta.unary.prop/is_scoped_enum.pass.cpp:1