[Sema] Warn on attribute nothrow conflicting with language specifiers

I discovered it was possible to create a 'nothrow' noexcept(false)
function, which is both non-sensical as well as seemingly breaking.

This patch warns if attribute nothrow is used with anything besides "noexcept".

"noexcept(true)" isn't possible, because the noexcept decl isn't parsed until
later.

Differential Revision: https://reviews.llvm.org/D38205

llvm-svn: 314461