Rewrite type compatibility testing to do type merging rather than just

testing compatibility.  This is necessary for some constructs, like merging
redeclarations.

Also, there are some ObjC changes to make sure that 
typesAreCompatible(a,b) == typesAreCompatible(b,a).  I don't have any 
ObjC code beyond the testsuite, so please tell me if there are any cases 
where this doesn't behave as expected.

llvm-svn: 55158