[AMDGPU] Add amdgpu_cs_chain[_preserve] CCs to IR & verifier

Add the amdgpu_cs_chain and amdgpu_cs_chain_preserve keywords to
LLVM IR and make sure we can parse and print them. Also make sure we
perform some basic checks in the IR verifier - similar to what we check
for many of the other AMDGPU calling conventions, plus the additional
restriction that we can't have direct calls to functions with these
calling conventions.

Differential Revision: https://reviews.llvm.org/D151994