[diagtool] Add a new "tree" command to shows warnings activated by a flag.

% diagtool tree -Wunused-value
-Wunused-value
  -Wunused-comparison
    warn_unused_comparison
  -Wunused-result
    warn_unused_result
  warn_unused_call
  warn_unused_container_subscript_expr
  warn_unused_expr
  warn_unused_property_expr
  warn_unused_voidptr

llvm-svn: 159093