[codeview] Cope with unsorted streams in type merging

Summary:
MASM can produce type streams that are not topologically sorted. It can
even produce type streams with circular references, but those are not
common in practice.

Reviewers: inglorion, ruiu

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31629

llvm-svn: 299403