[ObjC] Pick a 'readwrite' property when synthesizing ambiguous

property and check for incompatible attributes

This commit changes the way ambiguous property synthesis (i.e. when synthesizing
a property that's declared in multiple protocols) is performed. Previously,
Clang synthesized the first property that was found. This lead to problems when
the property was synthesized in a class that conformed to two protocols that
declared that property and a second protocols had a 'readwrite' declaration -
the setter was not synthesized so the class didn't really conform to the second
protocol and user's code would crash at runtime when they would try to set the
property.

This commit ensures that a first readwrite property is selected. This is a
semantic change that changes users code in this manner:

```
@protocol P @property(readonly) int p; @end
@protocol P2 @property(readwrite) id p; @end
@interface I <P2> @end
@implementation I
@syntesize p; // Users previously got a warning here, and Clang synthesized
              // readonly 'int p' here. Now Clang synthesizes readwrite 'id' p..
@end
```

To ensure that this change is safe, the warning about incompatible types is
promoted to an error when this kind of readonly/readwrite ambiguity is detected
in the @implementation. This will ensure that previous code that had this subtle
bug and ignored the warning now will fail to compile with an error, and users
should not get suprises at runtime once they resolve the error.

The commit also extends the ambiguity checker, and now it can detect conflicts
among the different property attributes. An error diagnostic is used for
conflicting attributes, to ensure that the user won't get "suprises" at runtime.

ProtocolPropertyMap is removed in favour of a a set + vector because the map's
order of iteration is non-deterministic, so it couldn't be used to select the
readwrite property.

rdar://31579994

Differential Revision: https://reviews.llvm.org/D35268


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@307903 91177308-0d34-0410-b5e6-96231b3b80d8