[Sema] Allow creating types with multiple of the same addrspace.

Summary:
The comment with the OpenCL clause about this clearly
says: "No type shall be qualified by qualifiers for
two or more different address spaces."

This must mean that two or more qualifiers for the
_same_ address space is allowed. However, it is
likely unintended by the programmer, so emit a
warning.

For dependent address space types, reject them like
before since we cannot know what the address space
will be.

Patch by Bevin Hansson (ebevhan).

Reviewers: Anastasia

Reviewed By: Anastasia

Subscribers: bader, cfe-commits

Differential Revision: https://reviews.llvm.org/D47630

llvm-svn: 335103