[CodeView] Add TypeDatabase class.

This creates a centralized class in which to store type records.
It stores types as an array of entries, which matches the
notion of a type stream being a topologically sorted DAG.
Logic to build up such a database was already being used in
CVTypeDumper, so CVTypeDumper is now updated to to read from
a TypeDatabase which is filled out by an earlier visitor in
the pipeline.

Differential Revision: https://reviews.llvm.org/D28486

llvm-svn: 291626