Firefox (I’ve seen bug reports from people trying this, but I’ve never tried it)
Notably, the first two of those projects don’t need RTTI, exceptions, or dllexport/dllimport of classes, which are the big gaps in Clang’s compatibility today.
Having some amount of fallback keeps us honest w.r.t. ABI compatibility, but these days we don’t fall back very often. Soon we’ll have to take a more aggressive approach of mixing and matching object files between MSVC and Clang before linking and testing the resulting binary.