[analyzer] Add minimal support for fix-it hints.
Allow attaching fixit hints to Static Analyzer BugReports.
Fixits are attached either to the bug report itself or to its notes
(path-sensitive event notes or path-insensitive extra notes).
Add support for fixits in text output (including the default text output that
goes without notes, as long as the fixit "belongs" to the warning).
Add support for fixits in the plist output mode.
Implement a fixit for the path-insensitive DeadStores checker. Only dead
initialization warning is currently covered.
Implement a fixit for the path-sensitive VirtualCall checker when the virtual
method is not pure virtual (in this case the "fix" is to suppress the warning
by qualifying the call).
Both fixits are under an off-by-default flag for now, because they
require more careful testing.
Differential Revision: https://reviews.llvm.org/D65182