Replace some unnecessary O(N^2) lookups for properties with
DeclContext lookups. The performance win is negligible in my tests,
but it's the Right Thing To Do (TM).



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173068 91177308-0d34-0410-b5e6-96231b3b80d8