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
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