[diagtool] Add diagtool to install target.

Although not very well known, diagtool is an incredibly convenient
utility for dealing with diagnostics.
Particularly useful are the "tree" and "show-enabled" commands:

 - The former prints the hierarchy of diagnostic (warning) flags and
   which of them are enabled by default.
 - The latter can be used to replace an invocation to clang and will
   print which diagnostics are disabled, warnings or errors.
   For instance: `diagtool show-enabled -Wall -Werror /tmp/test.c` will
   print that -Wunused-variable (warn_unused_variable) will be treated as
   an error.

This patch adds them to the install target so it gets shipped with the
LLVM release. It also adds a very basic man page and mentions this
change in the release notes.

Differential revision: https://reviews.llvm.org/D46694

llvm-svn: 332448