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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280549 91177308-0d34-0410-b5e6-96231b3b80d8