Speed up iteration of CodeView record streams.

There's some abstraction overhead in the underlying
mechanisms that were being used, and it was leading to an
abundance of small but not-free copies being made.  This
showed up on a profile.  Eliminating this and going back to
a low-level byte-based implementation speeds up lld with
/DEBUG between 10 and 15%.

Differential Revision: https://reviews.llvm.org/D42148

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@322871 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed