Remove vptr dispatch from FoldingSet.
Summary:
Instead of storing a vptr in each FoldingSet instance, form an
equivalent struct and pass it implicitly from FoldingSet into the
various FoldingSetBase methods.
This has three benefits:
* FoldingSet becomes one pointer smaller.
* Under LTO, the "virtual" functions are much easier to inline.
* The element type no longer needs to be complete when instantiating
FoldingSet<T>, only when instantiating an insert / lookup member.
Reviewers: rnk
Subscribers: hiraditya, dexonsmith, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D78247