PDB: Merge type info.

This patch is to merge type info in multiple .debug$T sections.

One mystery that needs to be solved is that it is not clear how
the MSVC linker uses TPI and IPI streams. Both streams contain
type info, and it is not obvious what kind of record should go
which.

dumppdb command in microsoft-pdb repository prints out IPI stream
contents as "IDs" and TPI stream as "TYPES", but looks like the tool
don't really care about which stream type recrods were read from.

For now, in this patch, I emit all type records to TPI stream.
It might just work with other tools. If not, we need to investigate
it more.

llvm-svn: 291739