[PDB/CodeView] Rename some classes.

In preparation for introducing writing capabilities for each of
these classes, I would like to adopt a Foo / FooRef naming
convention, where Foo indicates that the class can manipulate and
serialize Foos, and FooRef indicates that it is an immutable view of
an existing Foo.  In other words, Foo is a writer and FooRef is a
reader.  This patch names some existing readers to conform to the
FooRef convention, while offering no functional change.

llvm-svn: 301810