Include definitions behind __cplusplus macro (#55902)

Summary:
Pull Request resolved: #55902

Includes definitions behind the `__cplusplus` macro to also be visible in the API snapshot.

Changelog:
[Internal]

Reviewed By: cortinico

Differential Revision: D95189538

Author: coado

scripts/cxx-api/config.yml

@@ -61,6 +61,7 @@ ReactApple:
     - "*/platform/macos/*"
     - "*/platform/android/*"
   definitions:
+    __cplusplus: 1
   variants:
     debug:
       definitions:

scripts/cxx-api/tests/snapshots/.doxygen.config.template

@@ -2442,7 +2442,8 @@ INCLUDE_FILE_PATTERNS  =
 # recursively expanded use the := operator instead of the = operator.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.
 
-PREDEFINED             = FOLLY_PACK_PUSH="" \
+PREDEFINED             = __cplusplus=1 \
+                         FOLLY_PACK_PUSH="" \
                          FOLLY_PACK_POP="" \
                          FOLLY_PACK_ATTR="" \
                          __attribute__(x)="" \

scripts/cxx-api/tests/snapshots/should_handle_ifdef_cplusplus/snapshot.api

@@ -0,0 +1,7 @@
+category RCTBridgeProxy(Cxx) {
+  public @property (assign) int cxxOnlyProperty;
+}
+
+interface RCTBridgeProxy : public NSObject {
+  public @property (assign) int cxxOnlyProperty;
+}

scripts/cxx-api/tests/snapshots/should_handle_ifdef_cplusplus/test.h

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+@interface RCTBridgeProxy : NSObject
+
+@end
+
+@interface RCTBridgeProxy (Cxx)
+
+#ifdef __cplusplus
+@property (nonatomic, readwrite) int cxxOnlyProperty;
+#endif
+
+@end