Add CodeViewRecordIO for reading and writing.

Using a pattern similar to that of YamlIO, this allows
us to have a single codepath for translating codeview
records to and from serialized byte streams.  The
current patch only hooks this up to the reading of
CodeView type records.  A subsequent patch will hook
it up for writing of CodeView type records, and then a
third patch will hook up the reading and writing of
CodeView symbols.

Differential Revision: https://reviews.llvm.org/D26040

llvm-svn: 285836