Revert r188574.  Turns out it isn't needed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188578 91177308-0d34-0410-b5e6-96231b3b80d8
Replace some DenseMap keys with simpler structures that don't need another DenseMapInfo specialization.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188580 91177308-0d34-0410-b5e6-96231b3b80d8
RetainCountChecker: Replace some loops with std:: algorithms.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188581 91177308-0d34-0410-b5e6-96231b3b80d8