[Driver] Use platform-appropriate profiling libraries for WatchOS, TVOS

When adding profiling instrumentation, use libclang_rt.profile_tvos.a
for TVOS targets and libclang_rt.profile_watchos.a for WatchOS targets.

I've also fixed up a comment and added an assert() that prevents us from
defaulting to an incorrect platform.

Differential Revision: http://reviews.llvm.org/D14521

Reviewed-by: t.p.northover
llvm-svn: 252558