Implement Objective-C Related Result Type semantics.

Related result types apply Cocoa conventions to the type of message
sends and property accesses to Objective-C methods that are known to
always return objects whose type is the same as the type of the
receiving class (or a subclass thereof), such as +alloc and
-init. This tightens up static type safety for Objective-C, so that we
now diagnose mistakes like this:

t.m:4:10: warning: incompatible pointer types initializing 'NSSet *'
with an
      expression of type 'NSArray *' [-Wincompatible-pointer-types]
  NSSet *array = [[NSArray alloc] init];
         ^       ~~~~~~~~~~~~~~~~~~~~~~
/System/Library/Frameworks/Foundation.framework/Headers/NSObject.h:72:1:
note: 
      instance method 'init' is assumed to return an instance of its
      receiver
      type ('NSArray *')
- (id)init;
^

It also means that we get decent type inference when writing code in
Objective-C++0x:

  auto array = [[NSMutableArray alloc] initWithObjects:@"one",  @"two",nil];
  //    ^ now infers NSMutableArray* rather than id

llvm-svn: 132868

clang/lib/Frontend/CompilerInvocation.cpp

@@ -670,6 +670,9 @@ static void LangOptsToArgs(const LangOptions &Opts,
       Res.push_back("-fobjc-gc-only");
     }
   }
+  if (Opts.ObjCInferRelatedResultType)
+    Res.push_back("-fobjc-infer-related-result-type");
+  
   if (Opts.AppleKext)
     Res.push_back("-fapple-kext");
   
@@ -1485,6 +1488,9 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
   else if (Args.hasArg(OPT_fobjc_gc))
     Opts.setGCMode(LangOptions::HybridGC);
   
+  if (Args.hasArg(OPT_fobjc_infer_related_result_type))
+    Opts.ObjCInferRelatedResultType = 1;
+  
   if (Args.hasArg(OPT_fapple_kext)) {
     if (!Opts.CPlusPlus)
       Diags.Report(diag::warn_c_kext);