Sema: Add an access level to the fix-its for missing imports when appropriate.
When emitting fix-its for missing imports, include an access level when the
module has been imported with an access level in other source files. For now,
the suggested access level for will always be `internal`, even when uses of
members in the file would actually require `public` or `package` visibility. In
order to suggest the correct access level, name lookup will need to be
refactored to repair references to inaccessible declarations, instead of
leaving error nodes in the AST. In anticipation of that refactoring of name
lookup, missing import diagnostics are now delayed until type checking a source
file is finished so that a consistent access level can be suggested for each
import fix-it for a given module.
Partially resolves rdar://126637855.
swift
3ef16007 - Sema: Add an access level to the fix-its for missing imports when appropriate.
