Implement `with(auto x = ...)`

[thewilsonator]

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[dejlek]

Thank you for doing this!

[nordlow]

At https://buildkite.com/dlang/dmd/builds/44260/steps/canvas?sid=019afb40-c8cd-49d1-9a18-a711b8b866c0 seems to be a conflict with the syntax

tests/unit_threaded/ut/integration.d(24,20): Error: found `(` while expecting `=` or identifier
    with(immutable Sandbox()) {
                   ^

. The caret position seems to be off.