Adds VisibilityRegion, VisibilityCondition and the region table: print
once, mark what each display option controls, and project the frozen
result onto any option set to get what printing with those options would
have produced. Containers carry their items' conditions onto the line
breaks, separators and braces they print. MachOSwiftSection and
MachOObjCSection build their marking modes on it.

Also faster existential appends, result-builder accumulation and string
concatenation, with no behavior change (docs/AppendPathPerformance.md).