[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: https://llvm.org/svn/llvm-project/cfe/trunk@306559 91177308-0d34-0410-b5e6-96231b3b80d8