Since commit 474eafe28c (clang-cl: Add support for C++23, 2024-09-13,
v3.31.0-rc1~97^2) we use a Clang-specific flag to enable C++23 since
`clang-cl` has no `-std:c++23` flag, and `-std:c++latest` may enable an
even newer version of C++. However, in `.vcxproj` files there is no way
to express a target-wide `-clang:-std=c++23` flag for only C++ sources
when the target also has C sources. Add a special case to map back to
`-std:c++latest` for targets with C++23 and C together.
Fixes: #26508
30139913e9