* Add `lexically_normal` test for all platforms.
* On Windows, MinGW does not currently handle `lexically_normal()`
correctly on UNC path, but MSVC and IntelLLVM do--add a comment
on this to avoid future confusion.
* Add test with `\\?\` notation and `weakly_canonical` that also triggers
the MinGW bug, but is fine with MSVC and oneAPI, for a more robust and
comprehensive test.
The `remove_filename` and `replace_extension` methods compute an offset
between the whole path in a `std::string` and a part of a path in a
`std::string_view`. This is done by subtracting their `.data()`
pointers. However, C++17 adds a non-const `.data()` through which
modification of the string is allowed. This means the copy-on-write
implementation used by the pre-C++11 std::string GNU ABI must reallocate
if the string has been copied. Our subtraction then computes an offset
between two different allocations, which is undefined behavior.
The workaround in commit b3ca4f9ad1 (cm/filesystem: Work around crash
when compiled for CYGWIN/MSYS runtime, 2021-04-22, v3.21.0-rc1~271^2~2)
avoided the problem by calling the non-const `.data()` to reallocate
before constructing the `string_view`. Instead, explicitly call the
const `.data()` method on the string, which does not reallocate.
Fixes: #22090, #23328
