[YAMLTraits] Add filename support to yaml::Input

Summary:
The current yaml::Input constructor takes a StringRef of data as its
first parameter, discarding any filename information that may have been
present when a YAML file was opened. Add an alterate yaml::Input
constructor that takes a MemoryBufferRef, which can have a filename
associated with it. This leads to clearer diagnostic messages.

Sponsored By: DARPA, AFRL

Reviewed By: arphaman

Differential Revision: https://reviews.llvm.org/D35398

Patch by: Jonathan Anderson (trombonehero)

llvm-svn: 308172