[MSVC] Fix a warning that is a default-error on modern MSVC

[bob80905]

There are warnings emitted:

C:\actions-runner\_work\offload-test-suite\offload-test-suite\DXC\include\llvm/ADT/StringRef.h(582): error C2220: the following warning is treated  as an error                                                                                                                                         
C:\actions-runner\_work\offload-test-suite\offload-test-suite\DXC\include\llvm/ADT/StringRef.h(582): warning C5285: cannot declare a specialization for 'std::is_nothrow_constructible': Specializing this standard library template is forbidden by N5014 [meta.rqmts]/4                               
C:\Program Files\Microsoft Visual Studio\18\Community\VC\Tools\MSVC\14.51.36231\include\type_traits(1014): note: see declaration of                 'std::is_nothrow_constructible'                                                                                                                     
C:\actions-runner\_work\offload-test-suite\offload-test-suite\DXC\include\llvm/ADT/StringRef.h(582): note: to simplify migration, consider the      temporary use of /Wv:18 flag with the version of the compiler with which you used to build without warnings              

that can be remedied by guarding the specialization against the MSVC STL version that forbids it.
This PR adds an extra condition (!(defined(_MSVC_STL_UPDATE) && _MSVC_STL_UPDATE >= 202604L)) to skip the std::is_nothrow_constructible specializations on MSVC STL releases that enforce N5014 [meta.rqmts]/4. This is more precise than checking _MSC_VER because the restriction comes from the STL headers, not the compiler itself. Newer STL versions compute the trait correctly without the workaround.

The build fails because /WX treats the warning as an error (C2220).
Assisted by: Github Copilot
Resolves #8449