Re:Rejuvenating the Microsoft C/C++ Compiler
by cwyers on Friday September 25, 2015 @11:57AM
(#10278862)
Adobe is almost certainly one:
https://helpx.adobe.com/creative-suite/kb/microsoft-visual-c...
There's probably quite a few internal-only users of Visual C++ too.
EDIT: Well, they're a high-profile user of Visual C++, I have no idea if they require C99 support but they ship a Mac version so it's possible.
If I recall correctly it was at a Visual Studio event last year, on those floor interviews they publish with the teams on C9.