[codeview] Make FieldList records print as a yaml sequence.

Before we were kind of imitating the behavior of a Yaml sequence
by outputting each record one after the other.  This makes it a
little cumbersome when we want to go the other direction -- from
Yaml to Pdb.  So this treats FieldList records as no different than
any other list of records, by printing them as a Yaml sequence with
the exact same format.

llvm-svn: 280549