[pdb] Pass CVRecord's through the visitor as non-const references.

This simplifies a lot of code, and will actually be necessary for
an upcoming patch to serialize TPI record hash values.

The idea before was that visitors should be examining records, not
modifying them.  But this is no longer true with a visitor that
constructs a CVRecord from Yaml.  To handle this until now, we
were doing some fixups on CVRecord objects at a higher level, but
the code is really awkward, and it makes sense to just have the
visitor write the bytes into the CVRecord.  In doing so I uncovered
a few bugs related to `Data` and `RawData` and fixed those.

Reviewed By: rnk
Differential Revision: https://reviews.llvm.org/D24362

llvm-svn: 281067