[Assignment Tracking] Give -fexperimental-assignment-tracking flag 3 options

Without this patch assignment tracking is enabled with
`-fexperimental-assignment-tracking` and disabled with
`-fno-experimental-assignment-tracking` (default). This patch removes the
`-fno-` version and changes `-fexperimental-assignment-tracking` to take 3
values: `enabled`, `disabled` (default), and `forced`.

`clang -Xclang -fexperimental-assignment-tracking=enabled` enables the feature
if some other conditions are met and `=forced` enables it without any further
checks.

If `enabled` is specified the feature will remain disabled if any of the
following are true: it's an LTO or ThinLTO build, optimisations are not
enabled, or lldb debugger tuning has been specified. See this short RFC
for more info:
https://discourse.llvm.org/t/rfc-enable-assignment-tracking/69399

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D146615