Disable type merging with gc attributes. This has a number of nasty

properties at the moment:

1. It allows stuff like "__strong id x; __weak id x;".

2. For constructs like "__strong id x; id x;", subsequent references to 
x lose the objc_gc attribute.

3. This incorrectly allows merges involving the address_space attribute.

4. Constructs like "id x; /* some code using x */ __weak id x;" don't 
apply the objc_gc attribute consistently to all uses of x.

The first three can probably be fixed relatively easily; the fourth 
would be extremely difficult to fix.

llvm-svn: 72683