Reapply D146987 "[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).

This reverts commit 9e7f14a8 which reverts
https://reviews.llvm.org/D146987