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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@72683 91177308-0d34-0410-b5e6-96231b3b80d8