Make C++ requirement opt-in (facebook#27730)

Summary:
When building as a framework these headers get automatically added to the framework umbrella header for React-Core. Instead of converting all the React sources to ObjC++ files and still forcing external users that build native source (and link against a framework build) to also compile as ObjC++, this makes the attribution related methods that were added in facebook@fdcdca4 opt-in to ObjC++ builds.

This is also the reason for the current failure of the CI `test_ios_frameworks` run.

I’m unsure if this change really warrants an entry in the CHANGELOG, as it’s more of an amendment of the (afaik) unreleased [change](facebook@fdcdca4).

[iOS] [Fixed] - Make framework builds work again by making `RCTImageLoader` C++ requirement opt-in
Pull Request resolved: facebook#27730

Test Plan:
I tested static and dynamic (framework) builds and ran the test suite.

This change should make the `test_ios_frameworks` CI run _build_ again, ~~but it may still fail overall as in my local testing one of the tests leads to a segfault (which I will try to address separately)~~.

Reviewed By: PeteTheHeat

Differential Revision: D19348846

Pulled By: fkgozali

fbshipit-source-id: 8a74e6f7ad3ddce2cf10b080b9a5d7b399bd5fc0

Libraries/Image/RCTImageLoaderWithAttributionProtocol.h

@@ -12,6 +12,8 @@
 
 @protocol RCTImageLoaderWithAttributionProtocol<RCTImageLoaderProtocol>
 
+// TODO (T61325135): Remove C++ checks
+#ifdef __cplusplus
 /**
  * Same as the variant in RCTImageURLLoaderProtocol, but allows passing attribution
  * information that each image URL loader can process.
@@ -25,5 +27,6 @@
                                              progressBlock:(RCTImageLoaderProgressBlock)progressBlock
                                           partialLoadBlock:(RCTImageLoaderPartialLoadBlock)partialLoadBlock
                                            completionBlock:(RCTImageLoaderCompletionBlock)completionBlock;
+#endif
 
 @end

Libraries/Image/RCTImageURLLoaderWithAttribution.h

@@ -7,6 +7,8 @@
 
 #import <React/RCTImageURLLoader.h>
 
+// TODO (T61325135): Remove C++ checks
+#ifdef __cplusplus
 namespace facebook {
 namespace react {
 
@@ -17,13 +19,16 @@ struct ImageURLLoaderAttribution {
 
 } // namespace react
 } // namespace facebook
+#endif
 
 /**
  * Same as the RCTImageURLLoader interface, but allows passing in optional `attribution` information.
  * This is useful for per-app logging and other instrumentation.
  */
 @protocol RCTImageURLLoaderWithAttribution <RCTImageURLLoader>
 
+// TODO (T61325135): Remove C++ checks
+#ifdef __cplusplus
 /**
  * Same as the RCTImageURLLoader variant above, but allows optional `attribution` information.
  */
@@ -35,5 +40,6 @@ struct ImageURLLoaderAttribution {
                                    progressHandler:(RCTImageLoaderProgressBlock)progressHandler
                                 partialLoadHandler:(RCTImageLoaderPartialLoadBlock)partialLoadHandler
                                  completionHandler:(RCTImageLoaderCompletionBlock)completionHandler;
+#endif
 
 @end