[CMake] Creating add_clang_tool macro to wrap add_clang_executable and...

[CMake] Creating add_clang_tool macro to wrap add_clang_executable and generate install actions and targets.

This change brings forward the LLVM convention that "executables" are just runnable binaries, and "tools" are executables that are part of the project's install.

Having this abstraction will allow us to simplify some of the tool CMakeLists files, and it will standardize some of the install behaviors.

llvm-svn: 258209