Add 'diagtool' to the 'tools/' directory. diagtool is a new tool (WIP) for...

Add 'diagtool' to the 'tools/' directory.  diagtool is a new tool (WIP) for analyzing and working with clang diagnostics.

Some interesting stats from 'diagtool list-warnings' on the current version of clang:

  Percentage of warnings with flags: 48.79%
  Number of unique flags: 148
  Average number of diagnostics per flag: 2.041

llvm-svn: 137109