[ObjC] type method metadata `_imp`, messenger routine at callsite with program address space

On targets with non-default program address space (e.g., Harvard
architectures), clang crashes when emitting Objective-C method metadata,
because the address of the method IMP cannot be bitcast to i8*. It similarly
crashes at messenger callsite with a failed bitcast.

Define the _imp field instead as i8 addrspace(1)* (or whatever the target's
program address space is). And in getMessageSendInfo(), create signatureType by
specifying the program address space.

Add a regression test using the AVR target. Test failed previously and passes
now. Checked codegen of the test for x86_64-apple-darwin19.6.0 and saw no
difference, as expected.

Reviewed By: rjmccall, dylanmckay

Differential Revision: https://reviews.llvm.org/D112113