Reorganize some of the data formatters code to simplify CXXFormattersFunction.h. Also, add a synthetic child provider for libc++'s version of std::initializer_list<T>

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@220421 91177308-0d34-0410-b5e6-96231b3b80d8