This patch mainly consolidates the functions used to check accessors vs.
other decls, and makes sure we check setter access as well as regular
decl access.
rdar://45217648
Previously, members of protocols that were not protocol requirements,
like accessors and typealiases, did not inherit @usableFromInline from
the parent protocol. Change this so they do.
Static properties are not subject to the same restrictions as properties
whose initializers are exposed to clients in @_fixed_layout types.
rdar://45080912
...and collapse StaticVar/ClassVar and StaticLet/ClassLet into
StaticProperty/ClassProperty.
"var" and "let" aren't great nouns to use in diagnostics to begin with,
especially alongside semantic terms like "instance method". Focus on
the type vs. non-type aspect instead with "property", which better
matches how people talk about member vars (and lets) anyway.
Previously we made sure that inlinable function bodies only
reference @usableFromInline or public declarations, but we
also have to make sure that @usableFromInline declarations
only reference other declarations that are at least as
visible.
Otherwise, you could, for example, define a @usableFromInline
typealias which referenced an internal type, and then
reference the typealias from a @usableFromInline function body.
If the base class initializer was '@inlinable public' and the
derived class was '@usableFromInline internal', the synthesized
initializer was '@inlinable internal', which was an error.
