[codeview] YAMLize all section offsets and indices in symbol records

We forgot to serialize these because llvm-readobj didn't dump them. They
are typically all zeros in an object file. The linker fills them in with
relocations before adding them to the PDB. Now we can properly round
trip these symbols through pdb2yaml -> yaml2pdb.

I made these fields optional with a zero default so that we can elide
them from our test cases.

llvm-svn: 305857