clang-cl "/clang:" args not working with v17 build from main?

[Colengms]

I work on the C/C++ Extension for VS Code. We rely on the /clang: prefix in order to pass gcc args to clang-cl, to coax it into returning system include paths and system defines. i.e.: clang-cl.exe /clang:-xc++ /clang:-E /clang:-dM nul

In the process of assisting a dev working on Edge (Chromium), I discovered their process of building chromium involves snapping to a pre-release commit (in main, as per docs from Google), and that the latest builds from main no longer supports that syntax:

clang-cl.exe /clang:-xc++ /clang:-E /clang:-dM nul
clang-cl: error: unsupported option '-x c++'; did you mean '/TC' or '/TP'?

I'm not sure if this is a bug or intentional removal of this functionality. But, without this or an alternative, the C/C++ Extension for VS Code would be unable to support automatically querying system include paths and system defines from clang-cl. Could this get fixed/restored? By filing this issue, I'm hoping to head-off a large number of issues being opened against the C/C++ Extension for VS Code when LLVM 17 is released.

This is also how we support other compilers based on clang-cl, such as Intel's LLVM based compilers: icx.exe, dpcpp.exe, cpcpp-cl.exe

As a side note / bonus issue; We build clang-tidy and clang-format for distribution with the C/C++ Extension for VS Code. I recently tried to build these out of main. I noticed (at least as of commit f763270), that cross-compilation builds hosted on x64 macOS and targeting Arm64 macOS, were consistently hanging. This didn't repro when building from the version 16 release tag.

[Colengms]

It looks like our use of /clang:-xc++ was essentially being ignored in previous versions. Perhaps support for switching between C and C++ could be hooked up to /clang:-xc / /clang:-xc++ ? :)

I think we can work around this in the C/C+ Extension for VS Code by using /TC and /TP instead of /clang:-xc / /clang:-xc++.