[SyntaxArena] Remove fake AtomicBool for CMake builds

_SwiftSyntaxCShims is not part of CMake builds

Author: rintaro

Sources/SwiftSyntax/CMakeLists.txt

@@ -81,3 +81,7 @@ add_swift_syntax_library(SwiftSyntax
   generated/syntaxNodes/SyntaxNodesQRS.swift
   generated/syntaxNodes/SyntaxNodesTUVWXYZ.swift
 )
+
+target_link_swift_syntax_libraries(SwiftSyntax PUBLIC
+  _SwiftSyntaxCShims
+)

Sources/SwiftSyntax/SyntaxArena.swift

@@ -10,22 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if SWIFT_SYNTAX_BUILD_USING_CMAKE
-// The CMake bulid of swift-syntax does not build the _AtomicBool module because swift-syntax's CMake build is
-// Swift-only. Fake an `AtomicBool` type that is not actually atomic. This should be acceptable for the following
-// reasons:
-//  - `AtomicBool` is only used for the `hasParent` assertion, so release compilers don't rely on it
-//  - The compiler is single-threaded so it it is safe from race conditions on `AtomicBool`.
-fileprivate struct AtomicBool {
-  var value: Bool
-
-  init(initialValue: Bool) {
-    self.value = initialValue
-  }
-}
-#else
 import _SwiftSyntaxCShims
-#endif
 
 /// A syntax arena owns the memory for all syntax nodes within it.
 ///