Fix the libcxx set, multiset, vector and bitset formatters to work on references.

Fix the libcxx set, multiset, vector and bitset formatters to work on references.

The synthetic child providers for these classes had a type expression that matched
pointers & references to the type, but the Front End only worked on the actual object.

I fixed this by adding a way for the Synthetic Child FrontEnd provider to request dereference,
and then had these formatters use that mode.

<rdar://problem/40849836>

Differential Revision: https://reviews.llvm.org/D49279

llvm-svn: 337035