[PredicateInfo] Use custom mangling to support ssa_copy with unnamed types.

This is a workaround and it would be better to fix this generally, but
doing it generally is quite tricky. See D48541 and PR38117.

Doing it in PredicateInfo directly allows us to use the type address to
differentiate different unnamed types, because neither the created
declarations nor the ssa_copy calls should be visible after
PredicateInfo got destroyed.

Reviewers: efriedma, davide

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D49126

llvm-svn: 337828