[libclang] Make clang_getCursorCompletionString not depend on the ASTUnit having a Sema.
[libclang] Make clang_getCursorCompletionString not depend on the ASTUnit having
a Sema.

This allows it to work when Sema is not available, like when loading AST files.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@148279 91177308-0d34-0410-b5e6-96231b3b80d8

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