[analyzer] Put llvm.Conventions back in alpha

Interestingly, this many year old (when I last looked I remember 2010ish)
checker was committed without any tests, so I thought I'd implement them, but I
was shocked to see how I barely managed to get it working. The code is severely
outdated, I'm not even sure it has ever been used, so I'd propose to move it
back into alpha, and possibly even remove it.

Differential Revision: https://reviews.llvm.org/D53856

llvm-svn: 345990