Re-add "Make FieldList records print as a YAML sequence"

This was originally submitted in r280549, and reverted in r280577
due to breaking one MSVC buildbot.  The issue is that MSVC 2013
doesn't synthesize move constructors.  So even though i was
writing std::move(A) it was copying it, leading to a bogus ArrayRef.
The solution here is to simply remove the std::vector<> from the
type, since it is unused and unnecessary.  This way the ArrayRef
continues to point into the original memory backing the CVType.

llvm-svn: 280769