[ADT] Reduce duplication between {Contextual,}FoldingSet; NFC

This is a precursor to another change (coming soon) that aims to make
FoldingSet's API more type-safe. Without this, the type-safety change
would just duplicate 4 more public methods between the already very
similar classes.

This renames FoldingSetImpl to FoldingSetBase so it's consistent with
the FooBase -> FooImpl<T> -> Foo<T> convention we seem to have with
other containers.

llvm-svn: 305231