[codeview] Fix YAML for LF_TYPESERVER2 by hoisting PDB_UniqueId

Summary:
We were treating the GUIDs in TypeServer2Record as strings, and the
non-ASCII bytes in the GUID would not round-trip through YAML.

We already had the PDB_UniqueId type portably represent a Windows GUID,
but we need to hoist that up to the DebugInfo/CodeView library so that
we can use it in the TypeServer2Record as well as in PDB parsing code.

Reviewers: inglorion, amccarth

Subscribers: llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D35495

llvm-svn: 308234