[gicombiner] Add a CodeExpander to handle C++ fragments with variable expansion

Summary:
This will handle expansion of C++ fragments in the declarative combiner
including custom predicates, and escapes into C++ to aid the migration
effort.

Reviewers: bogner, volkan

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68288

llvm-svn: 373551