[OpenCL] Change diagnostic for function declaration

[svenvh]

Summary

When declaring functions only differing by their return type,
diagnostic was indicating "conflicting types for" the function.
Similarly to c++, it is now indicating that "functions that differ
only in their return type cannot be overloaded".

For this code example:

float test (float p) {return 2.;}
double test (float p) {return 2.;}

This is changing this diagnostic:

tmp.cl:5:37: error: conflicting types for 'tee'
float __attribute__((overloadable)) test (float p) {return 2.;}
                                    ^
tmp.cl:4:38: note: previous definition is here
double __attribute__((overloadable)) test (float p) {return 2.;}
                                     ^
1 error generated.

To this:

tmp.cl:5:37: error: functions that differ only in their return type cannot be overloaded
float __attribute__((overloadable)) test (float p) {return 2.;}
~~~~~                               ^
1 error generated.

This should maybe be extended to other languages.

[svenvh]

Abandoning after an offline discussion with @Anastasia. Overloading is not a feature of OpenCL C in the first place so adding complexity to Clang isn't really justifiable.

clang/lib/Sema/SemaDecl.cpp

       return false;
 
     // Fall through for conflicting redeclarations and redefinitions.
+  } else if (getLangOpts().OpenCL) {
+    QualType OldDeclaredReturnType = Old->getDeclaredReturnType();
+    QualType NewDeclaredReturnType = New->getDeclaredReturnType();
+
+    if (!Context.hasSameType(OldDeclaredReturnType, NewDeclaredReturnType) &&
+        canFullyTypeCheckRedeclaration(New, Old, NewDeclaredReturnType,
+                                       OldDeclaredReturnType)) {
+
+        Diag(New->getLocation(), diag::err_ovl_diff_return_type)
+            << New->getReturnTypeSourceRange();
+        return true;
+    }
   }
 
   // C: Function types need to be compatible, not identical. This handles