[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159093 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed