[analyzer] Extend bug reports with extra notes

These diagnostics are separate from the path-sensitive engine's path notes,
and can be added manually on top of path-sensitive or path-insensitive reports.

The new note diagnostics would appear as note:-diagnostic on console and
as blue bubbles in scan-build. In plist files they currently do not appear,
because format needs to be discussed with plist file users.

The analyzer option "-analyzer-config notes-as-events=true" would convert
notes to normal path notes, and put them at the beginning of the path.
This is a temporary hack to show the new notes in plist files.

A few checkers would be updated in subsequent commits,
including tests for this new feature.

Differential Revision: https://reviews.llvm.org/D24278

llvm-svn: 283092