Add support for configuration with clang-cl version 7 or older that don't support /clang: arguments

[sean-mcmanus]

See the thread at #10529 (comment)

[sean-mcmanus]

Unfortunately, this appears to be unsupportable -- unless someone knows of an equivalent of /clang: with clang 7, which there doesn't appear. to be.

However, you should be able to workaround the issue by setting your compilerPath in c_cpp_properties.json to a clang-cl 8 or newer compiler. Your code would still be built with clang 7 (and compile_commands.json would still reference that version of clang-cl), but setting that compilerPath will enable us to be query that compiler correctly....assuming there isn't some other issue.

[Colengms]

We use /clang: to pass-through down a couple gcc-like args to get the compiler to output its system includes and system defines. So what we're blocked on here is some/any way to query system includes and system defines from. Without that capability, the compiler path is not useful to us, and the user would have to add system includes and defines to their c_cpp_properties.json configuration directly, as well as set the IntelliSenseMode.

[emptyVoid]

@sean-mcmanus @Colengms
The following command yields the same output in version 7.0.0 as does /clang:-E /clang:-dM nul in version 8.0.1:
"C:\Program Files (x86)\LLVM\bin\clang-cl.exe" -Xclang -E -Xclang -dM /E -c dummy.cpp
I couldn't find a way to feed nul as input though.

[Colengms]

Reopening. It looks like it might be possible to make this work clang-cl 7 with -Xclang instead of /clang:.  Some of the GGC args we need to pass in don't appear to be accepted by -Xclang, such as bitness args (-m32, -m64) and compiler flags (clang:-fno-blocks). And it doesn't accept nul or /dev/null output, so we would need to create a dummy source to compile, in a directory we can write to.

[github-actions]

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