[pdb] Make YamlTypeDumperCallbacks reuse *this.

[pdb] Make YamlTypeDumperCallbacks reuse *this.

Previously we were making new instances of YamlTypeDumperCallbacks
in order to recurse down and serialize / deserialize nested
records such as field lists.  This meant you could not pass
context from a higher operation to a lower operation because
it would be using a new instance of the visitor callback
delegate.

YAMLIO library was updated to support context-sensitive mappings,
so now we can reuse the same instance of the visitor callback
delegate even for nested operations.

llvm-svn: 280978