-fms-extensions: Error out on #pragma init_seg

By ignoring this pragma with a warning, we're essentially miscompiling
the user's program.  WebKit / Blink use this pragma to disable dynamic
initialization and finalization of some static data, and running the
dtors crashes the program.

Error out for now, so that /fallback compiles the TU correctly with
MSVC.  This pragma should be implemented some time this month, and we
can remove this hack.