Add extern "C" to header process_shims.h

Previous, we would encounter missing symbol errors like:

          "_subprocess_vm_size()", referenced from:
              closure #1 () -> Swift.Int in variable initialization expression of Subprocess.(_pageSize in _EDE99368FF43462E0C75AF3C9D4D8F2D) : Swift.Int in AsyncBufferSequence.swift.o
           NOTE: found '__subprocess_vm_size' in process_shims.c.o, declaration possibly missing 'extern "C"'

We can reproduce this by running:

        xcrun swift build -Xswiftc -cxx-interoperability-mode=default --build-tests

Adding `extern "C"` fixes these issues.

Author: j-hui

Sources/_SubprocessCShims/include/process_shims.h

@@ -35,11 +35,18 @@
 #include <sys/wait.h>
 #endif // TARGET_OS_LINUX || TARGET_OS_FREEBSD
 
+#ifdef __cplusplus
+#define SWIFTSUBPROCESS_EXPORT extern "C"
+#else
+#define SWIFTSUBPROCESS_EXPORT extern
+#endif
+
 #if __has_include(<mach/vm_page_size.h>)
-vm_size_t _subprocess_vm_size(void);
+SWIFTSUBPROCESS_EXPORT vm_size_t _subprocess_vm_size(void);
 #endif
 
 #if TARGET_OS_MAC
+SWIFTSUBPROCESS_EXPORT
 int _subprocess_spawn(
     pid_t * _Nonnull pid,
     const char * _Nonnull exec_path,
@@ -54,6 +61,7 @@ int _subprocess_spawn(
 );
 #endif // TARGET_OS_MAC
 
+SWIFTSUBPROCESS_EXPORT
 int _subprocess_fork_exec(
     pid_t * _Nonnull pid,
     int * _Nonnull pidfd,
@@ -69,19 +77,20 @@ int _subprocess_fork_exec(
     int create_session
 );
 
-int _was_process_exited(int status);
-int _get_exit_code(int status);
-int _was_process_signaled(int status);
-int _get_signal_code(int status);
-int _was_process_suspended(int status);
+SWIFTSUBPROCESS_EXPORT int _was_process_exited(int status);
+SWIFTSUBPROCESS_EXPORT int _get_exit_code(int status);
+SWIFTSUBPROCESS_EXPORT int _was_process_signaled(int status);
+SWIFTSUBPROCESS_EXPORT int _get_signal_code(int status);
+SWIFTSUBPROCESS_EXPORT int _was_process_suspended(int status);
 
-void _subprocess_lock_environ(void);
-void _subprocess_unlock_environ(void);
-char * _Nullable * _Nullable _subprocess_get_environ(void);
+SWIFTSUBPROCESS_EXPORT void _subprocess_lock_environ(void);
+SWIFTSUBPROCESS_EXPORT void _subprocess_unlock_environ(void);
+SWIFTSUBPROCESS_EXPORT char * _Nullable * _Nullable _subprocess_get_environ(void);
 
-int _subprocess_pdkill(int pidfd, int signal);
+SWIFTSUBPROCESS_EXPORT int _subprocess_pdkill(int pidfd, int signal);
 
 #if TARGET_OS_UNIX && !TARGET_OS_FREEBSD
+SWIFTSUBPROCESS_EXPORT
 int _shims_snprintf(
     char * _Nonnull str,
     int len,
@@ -92,7 +101,7 @@ int _shims_snprintf(
 #endif
 
 #if TARGET_OS_LINUX
-int _pidfd_open(pid_t pid);
+SWIFTSUBPROCESS_EXPORT int _pidfd_open(pid_t pid);
 
 // P_PIDFD is only defined on Linux Kernel 5.4 and above
 // Define our value if it's not available
@@ -113,14 +122,14 @@ typedef unsigned long DWORD;
 typedef int BOOL;
 #endif
 
-BOOL _subprocess_windows_send_vm_close(DWORD pid);
-unsigned int _subprocess_windows_get_errno(void);
+SWIFTSUBPROCESS_EXPORT BOOL _subprocess_windows_send_vm_close(DWORD pid);
+SWIFTSUBPROCESS_EXPORT unsigned int _subprocess_windows_get_errno(void);
 
 /// Get the value of `PROC_THREAD_ATTRIBUTE_HANDLE_LIST`.
 ///
 /// This function is provided because `PROC_THREAD_ATTRIBUTE_HANDLE_LIST` is a
 /// complex macro and cannot be imported directly into Swift.
-DWORD_PTR _subprocess_PROC_THREAD_ATTRIBUTE_HANDLE_LIST(void);
+SWIFTSUBPROCESS_EXPORT DWORD_PTR _subprocess_PROC_THREAD_ATTRIBUTE_HANDLE_LIST(void);
 
 #endif