[dsymutil] Out-line the YAML serialization code. NFC

[dsymutil] Out-line the YAML serialization code. NFC

It will get a bit bigger in an upcoming commit. No need to have all
of that in the header.

Also move parseYAMLDebugMap() to the same place as the serialization
code. This way it will be able to share a private Context object with
it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239185 91177308-0d34-0410-b5e6-96231b3b80d8