[diagtool] Change default tree behavior to print only flags

This patch changes the default behavior of `diagtool tree` to only
display warning flags and not the internal warnings flags. The latter is
an implementation detail of the compiler and usually not what the users
wants.

Furthermore, flags that are enabled by default are now also printed in
green. Originally, this was only the case for the diagnostic names.

Differential revision: https://reviews.llvm.org/D37390

llvm-svn: 312546