Add support for Clang GNU frontend on Windows

With Qt Creator on Windows it’s currently possible to use Clang with the MSVC frontend (clang-cl.exe), or to use Clang with the GNU frontend (i.e. clang.exe/clang++.exe) without a Visual Studio environment. However it’s not possible to use the GNU frontend with a Visual Studio environment.

We’d like to request support for this, as using the GNU frontend enables features that are not available with the MSVC frontend (e.g. Address Sanitizer).

To do this, the standard Clang compiler configuration UI could gain an optional "initialization" option to specify the vcvarsall invocation like the "Default LLVM" compiler.

See also:
https://lists.qt-project.org/pipermail/qt-creator/2024-November/009331.html