[llvm-pdbdump] Add the ability to merge PDBs.

Merging PDBs is a feature that will be used heavily by
the linker.  The functionality already exists but does not
have deep test coverage because it's not easily exposed through
any tools.  This patch aims to address that by adding the
ability to merge PDBs via llvm-pdbdump.  It takes arbitrarily
many PDBs and outputs a single PDB.

Using this new functionality, a test is added for merging
type records.  Future patches will add the ability to merge
symbol records, module information, etc.

llvm-svn: 303389