[ScanDependencies] Make sure canImport resolution agrees with import

Fix the problem that when the only module can be found is an
invalid/out-of-date swift binary module, canImport and import statement
can have different view for if the module can be imported or not.

Now canImport will evaluate to false if the only module can be found for
name is an invalid swiftmodule, with a warning with the path to the
module so users will not be surprised by such behavior.

rdar://128876895

include/swift/AST/ModuleLoader.h

@@ -287,7 +287,7 @@ public:
   ///
   /// If a non-null \p versionInfo is provided, the module version will be
   /// parsed and populated.
-  virtual bool canImportModule(ImportPath::Module named,
+  virtual bool canImportModule(ImportPath::Module named, SourceLoc loc,
                                ModuleVersionInfo *versionInfo,
                                bool isTestableImport = false) = 0;