Make CNIOAtomics and CNIODarwin compatible with C++ interop (#3251)

### Motivation:

This fixes linker errors such as
```
@nonobjc __C.catmc_nio_atomic_uint_least32_t_create_with_existing_storage(Swift.UnsafeMutablePointer<__C.catmc_nio_atomic_uint_least32_t>, Swift.UInt32) -> () in NIOAtomic.swift.o
   NOTE: found '_catmc_nio_atomic_uint_least32_t_create_with_existing_storage' in c-nioatomics.c.o, declaration possibly missing 'extern "C"'
```
when building with Swift/C++ interoperability enabled.

### Modifications:

Wrapped the C declarations a couple of headers in `extern "C" { ... }`
blocks.

### Result:

The project builds successfully with Swift/C++ interop enabled.

Co-authored-by: Cory Benfield <[email protected]>

Author: egorzhdan

Sources/CNIOAtomics/include/CNIOAtomics.h

@@ -15,6 +15,10 @@
 #include <stdbool.h>
 #include <stdint.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #define DECLARE_ATOMIC_OPERATIONS(type, name)                                                                                         \
   struct catmc_atomic_ ## name;                                                                                                       \
                                                                                                                                       \
@@ -78,3 +82,7 @@ DECLARE_ATOMIC_OPERATIONS(intptr_t, intptr_t)
 DECLARE_ATOMIC_OPERATIONS(uintptr_t, uintptr_t)
 
 #undef DECLARE_ATOMIC_OPERATIONS
+
+#ifdef __cplusplus
+}  /* extern "C" */
+#endif

Sources/CNIODarwin/include/CNIODarwin.h

@@ -24,6 +24,10 @@
 #include <sys/stat.h>
 #include <fts.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 // Darwin platforms do not have a sendmmsg implementation available to them. This C module
 // provides a shim that implements sendmmsg on top of sendmsg. It also provides a shim for
 // recvmmsg, but does not actually implement that shim, instantly throwing errors if called.
@@ -60,5 +64,9 @@ extern const unsigned long CNIODarwin_IOCTL_VM_SOCKETS_GET_LOCAL_CID;
 
 const char* CNIODarwin_dirent_dname(struct dirent* ent);
 
+#ifdef __cplusplus
+}  /* extern "C" */
+#endif
+
 #endif  // __APPLE__
 #endif  // C_NIO_DARWIN_H