[YAMLIO] Add the ability to map with context.

mapping a yaml field to an object in code has always been
a stateless operation.  You could still pass state by using the
`setContext` function of the YAMLIO object, but this represented
global state for the entire yaml input.  In order to have
context-sensitive state, it is necessary to pass this state in
at the granularity of an individual mapping.

This patch adds support for this type of context-sensitive state.
You simply pass an additional argument of type T to the
`mapRequired` or `mapOptional` functions, and provided you have
specialized a `MappingContextTraits<U, T>` class with the
appropriate mapping function, you can pass this context into
the mapping function.

Reviewed By: chandlerc
Differential Revision: https://reviews.llvm.org/D24162

llvm-svn: 280977