[PDB] Split item and type records when merging type streams

[PDB] Split item and type records when merging type streams

Summary: MSVC does this when producing a PDB.

Reviewers: ruiu

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31316

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298717 91177308-0d34-0410-b5e6-96231b3b80d8