[wasm] Annotate errno as SwiftPrivate by apinotes

This patch adds an apinotes file for SwiftWASILibc clang module to mark
`errno` macro hidden from Swift code. This resolves ambiguity between
the C macro definition and the Swift wrapper in WASILibc overlay module.

This change installs the apinotes file to the resource directories for
both lib/swift/apinotes and lib/swift_static/apinotes.

stdlib/public/Platform/CMakeLists.txt

@@ -508,6 +508,34 @@ if("WASI" IN_LIST SWIFT_SDKS)
                                  DESTINATION "lib/swift_static/${arch_subdir}"
                                  COMPONENT sdk-overlay)
     endif()
+
+    set(wasilibc_apinotes_source "SwiftWASILibc.apinotes")
+    add_custom_command_target(
+      copy_wasilibc_apinotes_resource
+      COMMAND
+        "${CMAKE_COMMAND}" "-E" "make_directory" ${SWIFTLIB_DIR}/apinotes ${SWIFTSTATICLIB_DIR}/apinotes
+      COMMAND
+        "${CMAKE_COMMAND}" "-E" "copy_if_different"
+          "${CMAKE_CURRENT_SOURCE_DIR}/${wasilibc_apinotes_source}" ${SWIFTLIB_DIR}/apinotes
+      COMMAND
+        "${CMAKE_COMMAND}" "-E" "copy_if_different"
+          "${CMAKE_CURRENT_SOURCE_DIR}/${wasilibc_apinotes_source}" ${SWIFTSTATICLIB_DIR}/apinotes
+      OUTPUT
+        ${SWIFTLIB_DIR}/apinotes/${wasilibc_apinotes_source}
+        ${SWIFTSTATICLIB_DIR}/apinotes/${wasilibc_apinotes_source}
+      COMMENT "Copying WASILibc API notes to resource directories")
+
+    list(APPEND wasilibc_modulemap_target_list ${copy_wasilibc_apinotes_resource})
+    add_dependencies(sdk-overlay ${copy_wasilibc_apinotes_resource})
+    swift_install_in_component(FILES "${wasilibc_apinotes_source}"
+                               DESTINATION "lib/swift/apinotes"
+                               COMPONENT sdk-overlay)
+    if(SWIFT_BUILD_STATIC_STDLIB)
+      swift_install_in_component(FILES "${wasilibc_apinotes_source}"
+                                 DESTINATION "lib/swift_static/apinotes"
+                                 COMPONENT sdk-overlay)
+    endif()
+
   endforeach()
 endif()
 add_custom_target(wasilibc_modulemap DEPENDS ${wasilibc_modulemap_target_list})

stdlib/public/Platform/SwiftWASILibc.apinotes

@@ -0,0 +1,5 @@
+Name: SwiftWASILibc
+Globals:
+  # errno macro is importable but we provide explicit Swift wrapper
+  - Name: errno
+    SwiftPrivate: true

test/stdlib/WASILibcAPI.swift

@@ -0,0 +1,19 @@
+// RUN: %target-swift-frontend -typecheck -swift-version 6 %s -verify
+// REQUIRES: executable_test
+// REQUIRES: OS=wasi
+
+import WASILibc
+
+// errno is a global thread-local variable, so it should be accessible
+// from any context.
+
+enum TestErrno {
+  static func testSyncContext() {
+    _ = errno
+    errno = 0
+  }
+  static func testAsyncContext() async {
+    _ = errno
+    errno = 0
+  }
+}