Amend formatting of `Substring.utf8Span` example code.
Use DocC tables in `Unicode.UTF8.ValidationError` overview.
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Co-authored-by: Alex Martini <amartini@apple.com>
The pass that annotated control-flow positions unreachable after an
infinite loop was migrated to pure Swift in PR 79186
(https://github.com/swiftlang/swift/pull/79186). As a result, the
C++-only bootstrap compiler is unable to determine that the
code-location is unreachable. Placing a fatalError after the infinite
while loop.
Fixes: rdar://149631113
