swift234

Push operator lookup into SourceFile, and simplify the interface.

The operator lookup cache already lived in SourceFile, but we need to be
able to look up operators on a per-SourceFile basis. Different files can
have different imports. The interface previously distinguished between
"no operator found" and "error", but none of the call sites were making
use of this distinction, and indeed some were misusing the return value
(Optional<SomeOperatorDecl *>). Now the lookup functions just return
operator decl pointers, which may be null.

Swift SVN r9668