Support ISO 9899:2018 (C17/C18)

[eternaleye]

C18 adds (almost) no features, and consists only of folding in Technical Corrigenda (and otherwise resolving defect reports against C11).

As a result, it should hopefully not be too onerous to support, and may also help further inform some places where judgment was required in interpreting the C11 standard.

The sole change beyond resolving DRs (listed in the summary linked below) is that ATOMIC_VAR_INIT is deprecated and no longer mandatory when initializing atomic variables.

References:

• The last public draft
• The clarification request summary for C11, which lists the ones which made it in (marked "C17")
• A post that states an intent to go into more detail on some of the changes