Bug 21782 – Add version variants for all cmd-line option checks

Using DMD command-line options,[1] one can (en/dis)able certain checks. Using a version statement, one can conditionally compile code depending on *some* of these options. D should provide a version identifier (or keyword) for *every* check. That identifier or keyword is exactly what it is in [1], and additionally, for every version identifier/keyword stated, another equivalent identifier should be available that starts with "check", in particular, I propose the following names: checkAsserts, checkBounds, checkInContracts, checkInvariants, checkOutContracts, checkFinalSwitch. [1] https://dlang.org/dmd-windows.html#switch-check

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