Don't call getMostRecentDecl when we know we have it.
Don't call getMostRecentDecl when we know we have it.

On a Release build this takes the testcase in pr18055 from 0m3.892s to 0m1.452s.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@195768 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang