Convert RCTConvert to mm in preparation for DisplayP3 changes

Summary:
This is a preliminary change which converts RCTConvert to an objectiveC++ file. This is required by the next diffs in the stack.

## Changelog
[iOS][Changed] - Make RCTConvert an Objective-C++ (`.mm`) file in prep for DisplayP3 changes

Reviewed By: javache

Differential Revision: D53520228

fbshipit-source-id: cf45c42955401b4e14fe68221129077817b2598e

packages/react-native/React/Base/RCTConvert.m → packages/react-native/React/Base/RCTConvert.mm

@@ -439,7 +439,8 @@ + (UIKeyboardType)UIKeyboardType:(id)json RCT_DYNAMIC
     mapping = temporaryMapping;
   });
 
-  UIKeyboardType type = RCTConvertEnumValue("UIKeyboardType", mapping, @(UIKeyboardTypeDefault), json).integerValue;
+  UIKeyboardType type =
+      (UIKeyboardType)RCTConvertEnumValue("UIKeyboardType", mapping, @(UIKeyboardTypeDefault), json).integerValue;
   return type;
 }
 
@@ -844,7 +845,7 @@ + (UIEdgeInsets)UIEdgeInsets:(id)json
     RCTAssert([UIColor respondsToSelector:selector], @"RCTUIColor does not respond to a semantic color selector.");
     Class klass = [UIColor class];
     IMP imp = [klass methodForSelector:selector];
-    id (*getSemanticColorObject)(id, SEL) = (void *)imp;
+    id (*getSemanticColorObject)(id, SEL) = (id(*)(id, SEL))imp;
     id colorObject = getSemanticColorObject(klass, selector);
     if ([colorObject isKindOfClass:[UIColor class]]) {
       color = colorObject;