ClangImporter: support SDK for APINotes

lib/ClangImporter/ClangImporter.cpp

@@ -749,10 +749,24 @@ void importer::getNormalInvocationArguments(
   invocationArgStrs.push_back("-fapinotes-modules");
   invocationArgStrs.push_back("-fapinotes-swift-version=" +
                               languageVersion.asAPINotesVersionString());
-  invocationArgStrs.push_back("-iapinotes-modules");
-  invocationArgStrs.push_back((llvm::Twine(searchPathOpts.RuntimeResourcePath) +
-                               llvm::sys::path::get_separator() +
-                               "apinotes").str());
+
+  // Prefer `-sdk` paths.
+  if (!searchPathOpts.getSDKPath().empty()) {
+    llvm::SmallString<261> path{searchPathOpts.getSDKPath()};
+    llvm::sys::path::append(path, "usr", "lib", "swift", "apinotes");
+
+    invocationArgStrs.push_back("-iapinotes-modules");
+    invocationArgStrs.push_back(path.str().str());
+  }
+
+  // Fallback to "legacy" `-resource-dir` paths.
+  {
+    llvm::SmallString<261> path{searchPathOpts.RuntimeResourcePath};
+    llvm::sys::path::append(path, "apinotes");
+
+    invocationArgStrs.push_back("-iapinotes-modules");
+    invocationArgStrs.push_back(path.str().str());
+  }
 }
 
 static void

test/ClangImporter/sdk-apinotes.swift

@@ -0,0 +1,6 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck %s -verify
+
+import cfuncs
+
+let array: [UnsafePointer<CChar>?] = [nil]
+array.withUnsafeBufferPointer { nullability_note($0.baseAddress!) }

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"