JIT: emit debug info for untracked locals when optimizing

[AndyAyersMS]

Per notes in #11239 -- when the jit is optimizing it relies on liveness to drive user var debug info emission. But since untracked user vars don't have liveness they do produce any debug representation.

Seems like a better optimized code debug experience could be offered by emitting these untracked user vars as live throughout the method. VS2017 looks like it does a decent job of trimming down over-reported live scopes to relevant code ranges.

category:implementation
theme:debug-info
skill-level:intermediate
cost:medium

[BruceForstall]

@AndyAyersMS you write:

But since untracked user vars don't have liveness they do produce any debug representation.
do you mean "do not produce"?

Do we just need to get USING_VARIABLE_LIVE_RANGE enabled?