This adds a visitor that is capable of accessing type
records randomly and caching intermediate results that it
learns about during partial linear scans. This yields
amortized O(1) access to a type stream even though type
streams cannot normally be indexed.
Differential Revision: https://reviews.llvm.org/D33009
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302936 91177308-0d34-0410-b5e6-96231b3b80d8