mark CAtomic import as internal or implementation-only (#200)

fixes rdar://134894555, fixes #195

Author: QuietMisdreavus

Sources/Markdown/Utility/AtomicCounter.swift

@@ -8,7 +8,11 @@
  See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 */
 
-import CAtomic
+#if compiler(>=6.0)
+internal import CAtomic
+#else
+@_implementationOnly import CAtomic
+#endif
 
 /// A wrapper for a 64-bit unsigned atomic singleton counter.
 struct AtomicCounter {