Add an option to suppress include stack printing on note diagnostics.

These stacks are often less important than those on primary diagnostics.

As the number of notes grows, this becomes increasingly important. The
include stack printing is clever and doesn't print stacks for adjacent
diagnostics from the same file, but when a note is in between a sequence
of errors in a header file, and the notes all refer to some other file,
we end up getting a worst-case ping-pong of include stacks that take up
a great deal of vertical space.

Still, for now, the default behavior isn't changed. We can evaluate user
feedback with the flag.

Patch by Richard Trieu, a couple of style tweaks from me.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@128371 91177308-0d34-0410-b5e6-96231b3b80d8