[Threading] Remove thread_get_main().

Removing thread_get_main() means we don't need a static initializer on
Darwin, which means we can delete Darwin.cpp as well.  We can also delete
Nothreads.cpp while we're about it, because there's nothing in that file.

rdar://90776105

Author: al45tair

include/swift/Threading/Impl/C11.h

@@ -49,7 +49,6 @@ do {                                                                    \
 using thread_id = ::thrd_t;
 
 inline thread_id thread_get_current() { return ::thrd_current(); }
-thread_id thread_get_main();
 bool thread_is_main();
 inline bool threads_same(thread_id a, thread_id b) {
   return ::thrd_equal(a, b);

include/swift/Threading/Impl/Darwin.h

@@ -32,8 +32,7 @@ using thread_id = ::pthread_t;
 
 inline thread_id thread_get_current() { return ::pthread_self(); }
 
-thread_id thread_get_main();
-bool thread_is_main();
+inline bool thread_is_main() { return pthread_main_np(); }
 
 inline bool threads_same(thread_id a, thread_id b) {
   return ::pthread_equal(a, b);

include/swift/Threading/Impl/Linux.h

@@ -55,7 +55,6 @@ using thread_id = ::pthread_t;
 
 inline thread_id thread_get_current() { return ::pthread_self(); }
 
-thread_id thread_get_main();
 bool thread_is_main();
 
 inline bool threads_same(thread_id a, thread_id b) {

include/swift/Threading/Impl/Nothreads.h

@@ -25,7 +25,6 @@ namespace threading_impl {
 using thread_id = unsigned;
 
 inline thread_id thread_get_current() { return 0; }
-inline thread_id thread_get_main() { return 0; }
 inline bool thread_is_main() { return true; }
 inline bool threads_same(thread_id a, thread_id b) { return a == b; }
 

include/swift/Threading/Impl/Pthreads.h

@@ -54,7 +54,6 @@ using thread_id = ::pthread_t;
 
 inline thread_id thread_get_current() { return ::pthread_self(); }
 
-thread_id thread_get_main();
 bool thread_is_main();
 
 inline bool threads_same(thread_id a, thread_id b) {

include/swift/Threading/Impl/Win32.h

@@ -29,7 +29,6 @@ namespace threading_impl {
 using thread_id = ::DWORD;
 
 inline thread_id thread_get_current() { return ::GetCurrentThreadId(); }
-thread_id thread_get_main();
 bool thread_is_main();
 inline bool threads_same(thread_id a, thread_id b) { return a == b; }
 

include/swift/Threading/Thread.h

@@ -55,11 +55,6 @@ class Thread {
     return Thread(threading_impl::thread_get_current());
   }
 
-  /// Returns the main thread in this program
-  static Thread main() {
-    return Thread(threading_impl::thread_get_main());
-  }
-
   /// Returns true iff executed on the main thread
   static bool onMainThread() {
     return threading_impl::thread_is_main();

lib/Threading/C11.cpp

@@ -64,14 +64,9 @@ C11ThreadingHelper helper;
 using namespace swift;
 using namespace threading_impl;
 
-thread_id
-swift::threading_impl::thread_get_main() {
-  return helper.main_thread();
-}
-
 bool
 swift::threading_impl::thread_is_main() {
-  return thrd_equal(thrd_current(), thread_get_main());
+  return thrd_equal(thrd_current(), helper.main_thread());
 }
 
 void

lib/Threading/CMakeLists.txt

@@ -1,7 +1,8 @@
+# If you update this, you also need to update the CMakeLists.txt file in
+# stdlib/public/Threading
+
 add_swift_host_library(swiftThreading STATIC
   C11.cpp
-  Darwin.cpp
   Linux.cpp
-  Nothreads.cpp
   Pthreads.cpp
   Win32.cpp)