Allow building swift-syntax in C++ interop mode

If swift-syntax is being built with `-cxx-interoperability-mode=default`, it fails because of `cannot initialize a variable of type 'AtomicBool *' with an rvalue of type 'void *'`. Add a cast here to fix the build issue.

rdar://129252735

Author: ahoppen

Sources/_SwiftSyntaxCShims/include/AtomicBool.h

@@ -21,7 +21,7 @@ typedef struct {
 } AtomicBool;
 
 static inline AtomicBool *_Nonnull swiftsyntax_atomic_bool_create(bool initialValue) {
-  AtomicBool *atomic = malloc(sizeof(AtomicBool));
+  AtomicBool *atomic = (AtomicBool *)malloc(sizeof(AtomicBool));
   atomic->value = initialValue;
   return atomic;
 }