[Concurrency] Fixes from initial review.

Rename `DispatchTaskExecutor` to `DispatchGlobalTaskExecutor` as we
may want to use the former for an executor that runs things on an
arbitrary Dispatch queue.

Rename `DispatchExecutor` to `DispatchExecutorProtocol`; again, we
might want the name for something else.

Add `@Sendable` attribute to `registerEvent`.

Fix missing `extern "C" SWIFT_CC(swift)` on `_swift_exit` (merge
error).

Remove stray whitespace from `CMakeLists.txt`

rdar://141348916

stdlib/public/Concurrency/ExecutorBridge.cpp

@@ -22,6 +22,7 @@ using namespace swift;
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wreturn-type-c-linkage"
 
+extern "C" SWIFT_CC(swift)
 void _swift_exit(int result) {
   exit(result);
 }