[analyzer] Add support for the note diagnostic pieces to plist output format.

Note diagnostic pieces are an additional way of highlighting code sections to
the user. They aren't part of the normal path diagnostic sequence. They can
also be attached to path-insensitive reports.

Notes are already supported by the text output and scan-build.

Expanding our machine-readable plist output format to be able to represent notes
opens up the possibility for various analyzer GUIs to pick them up.

Patch by Umann Kristóf!

Differential Revision: https://reviews.llvm.org/D45407

llvm-svn: 330766