[libclang] NFC, simplify clang_Cursor_Evaluate
[libclang] NFC, simplify clang_Cursor_Evaluate

Take advantage of early returns as suggested by Duncan in
https://reviews.llvm.org/D49051


git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@336591 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 7a9552d9f1a0e003326fa99215e050e75556127b)

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang