[diagtool] Install diagtool when LLVM_INSTALL_TOOLCHAIN_ONLY is ON.

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

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D80770