Not sure about other platforms but install-xcode-toolchain was already
including diagtool in the toolchain. This change makes it possible to
install diagtool during Apple's 2-stage build.
Instead of dropping if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY) conditional
I've switched to add_clang_tool which handles install targets. Also a
few other clang tools like clang-format, clang-scan-deps are using this
macro, so it is good to be consistent.
rdar://problem/15386909