Closure allocated in TokenizerBackedParser.PushSpanContextConfig

[ToddGrun]

Found while investigating allocations in the completion speedometer test. @333fred -- @DustinCampbell mentioned this might be going away soon, if so, then I guess this bug can just be closed.

[333fred]

I don't expect this to be going away any time soon, no.

[chsienki]

@333fred Will this be gone with the lexer changes, or still be around?

[333fred]

This should be handled with the lexer changes.