Switch to the appropriate C calling convention for pthread_main_np

Author: DougGregor

Runtimes/Core/Concurrency/CMakeLists.txt

@@ -112,6 +112,7 @@ target_compile_options(swift_Concurrency PRIVATE
   # NOTE: do not remove until `IsolatedAny` is on by default in all supported
   # compilers.
   "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-enable-experimental-feature IsolatedAny>"
+  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-enable-experimental-feature Extern>"
   # NOTE: enable the async frame pointer on Darwin to faciliate debugging.
   $<$<AND:$<PLATFORM_ID:Darwin>,$<COMPILE_LANGUAGE:C,CXX>>:-fswift-async-fp=always>
   "$<$<AND:$<PLATFORM_ID:Darwin>,$<COMPILE_LANGUAGE:Swift>>:SHELL:-Xfrontend -swift-async-frame-pointer=always>"

stdlib/public/Concurrency/CMakeLists.txt

@@ -68,6 +68,8 @@ list(APPEND SWIFT_RUNTIME_CONCURRENCY_SWIFT_FLAGS
 list(APPEND SWIFT_RUNTIME_CONCURRENCY_SWIFT_FLAGS "-strict-memory-safety")
 list(APPEND SWIFT_RUNTIME_CONCURRENCY_SWIFT_FLAGS "-enable-experimental-feature" "AllowUnsafeAttribute")
 
+list(APPEND SWIFT_RUNTIME_CONCURRENCY_SWIFT_FLAGS "-enable-experimental-feature" "Extern")
+
 list(APPEND SWIFT_RUNTIME_CONCURRENCY_C_FLAGS
   "-D__STDC_WANT_LIB_EXT1__=1")
 

stdlib/public/Concurrency/MainActor.swift

@@ -166,7 +166,7 @@ extension MainActor {
 
 
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS) || os(visionOS)
-@_silgen_name("pthread_main_np")
+@_extern(c, "pthread_main_np")
 @usableFromInline
 internal func pthread_main_np() -> CInt