ClangImporter: support SDK for APINotes

Honour the SDK for APINotes to augment the system libraries. This allows
us to distribute APINotes with the Swift SDK and impact the system
without having to map the APINotes into the filesystem.

test/Inputs/clang-importer-sdk/usr/include/cfuncs.h

@@ -60,3 +60,5 @@ void opaque_pointer_param(struct not_importable *);
 
 int unsupported_parameter_type(int param1, _Complex int param2);
 _Complex int unsupported_return_type();
+
+void nullability_note(const char * _Nonnull const * _Nonnull parameter);

test/Inputs/clang-importer-sdk/usr/lib/swift/apinotes/cfuncs.apinotes

@@ -0,0 +1,7 @@
+---
+Name: cfuncs
+Functions:
+- Name: nullability_note
+  Parameters:
+  - Position: 0
+    Type: "const char * _Nullable const * _Nonnull"