[clang-reorder-fields] Switch to add_clang_tool

`add_clang_tool` invokes `add_clang_executable` internally, but it also
takes care of setting up the install rule. It also adds an `install-*`
build target, which is required for `LLVM_DISTRIBUTION_COMPONENTS`.

Differential Revision: https://reviews.llvm.org/D39523

llvm-svn: 317149