[Assignment Tracking] Enable by default

See https://discourse.llvm.org/t/rfc-enable-assignment-tracking/69399

This sets the `-Xclang -fexperimental-assignment-tracking` flag to the value
`enabled` which means it will be enabled so long as none of the following are
true: it's an LTO build, LLDB debugger tuning has been specified, or it's an O0
build (no work is done in any case if -g is not specified or -gmlt is used).

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D146987