Clang IntelliSense on Windows needs to be a combination of MSVC and Clang modes

[Colengms]

The following is from #3906:

clang-cl.exe should be treated like clang-x64 mode with -fms-extensions -- and possibly other config changes that we may not be handling currently. I believe originally, we expected clang-cl.exe to run with msvc-x64 mode (i.e. treated exactly like cl.exe, with no system include/define querying).

Currently, we are always switching to MSVC IntelliSense mode for clang.exe or clang-cl.exe, on Windows (whenever we see "Windows Kits" paths returned by the compiler when probed, which appears to be always). Clang enables some MSVC compatibility (required to compile windows headers?). Unfortunately, not using a Clang IntelliSense mode will cause squiggles for valid use of clang/gcc language extensions.

This issue is to track configuring IntelliSense for clang/clang-cl on Windows, such that the language features of the compiler are accurately reflected (both MSVC and Clang/gcc extensions).

[Colengms]

This is a dupe of: #3387