Add a static_assert to enforce that parameters to llvm::format() are not totally unsafe

Summary:
I had for the second time today a bug where llvm::format("%s", Str)
was called with Str being a StringRef. The Linux and MacOS bots were
fine, but windows having different calling convention, it printed
garbage.

Instead we can catch this at compile-time: it is never expected to
call a C vararg printf-like function with non scalar type I believe.

Reviewers: bogner, Bigcheese, dexonsmith

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25266

llvm-svn: 283509