Refactor the PDB Stream reading interface.

Refactor the PDB Stream reading interface.

The motivation for this change is that PDB has the notion of
streams and substreams.  Substreams often consist of variable
length structures that are convenient to be able to treat as
guaranteed, contiguous byte arrays, whereas the streams they
are contained in are not necessarily so, as a single stream
could be spread across many discontiguous blocks.

So, when processing data from a substream, we want to be able
to assume that we have a contiguous byte array so that we can
cast pointers to variable length arrays and such.

This leads to the question of how to be able to read the same
data structure from either a stream or a substream using the
same interface, which is where this patch comes in.

We separate out the stream's read state from the underlying
representation, and introduce a `StreamReader` class.  Then
we change the name of `PDBStream` to `MappedBlockStream`, and
introduce a second kind of stream called a `ByteStream` which is
simply a sequence of contiguous bytes.  Finally, we update all
of the std::vectors in `PDBDbiStream` to use `ByteStream` instead
as a proof of concept.

llvm-svn: 268071