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