[Bash-autocompletion] Invoke clang where user called
[Bash-autocompletion] Invoke clang where user called

Summary:
When user build clang and used completion Eg. `build/bin/clang -fno[tab]`, we want to invoke `build/bin/clang --autocomplete=-fno`, rather than `clang --autocomplete=-fno`.

Differential Revision: https://reviews.llvm.org/D34761

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@306559 91177308-0d34-0410-b5e6-96231b3b80d8

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