Improve C API support for atomicrmw and cmpxchg.

atomicrmw and cmpxchg have a volatile flag, so allow them to be get and set with LLVM{Get,Set}Volatile. atomicrmw and fence have orderings, so allow them to be get and set with LLVM{Get,Set}Ordering. Add missing LLVMAtomicRMWBinOpFAdd and LLVMAtomicRMWBinOpFSub enum constants. AtomicCmpXchg also has a weak flag, add a getter/setter for that too. Add a getter/setter for the binary-op of an atomicrmw.

atomicrmw and cmpxchg have a volatile flag, so allow it to be set/get with LLVMGetVolatile and LLVMSetVolatile. Add missing LLVMAtomicRMWBinOpFAdd and LLVMAtomicRMWBinOpFSub enum constants. AtomicCmpXchg also has a weak flag, add a getter/setter for that too. Add a getter/setter for the binary-op of an atomicrmw.

Add LLVMIsA## for CatchSwitchInst, CallBrInst and FenceInst, as well as AtomicCmpXchgInst and AtomicRMWInst.

Update llvm-c-test to include atomicrmw and fence, and to copy volatile for the four applicable instructions.

Differential Revision: https://reviews.llvm.org/D67132

llvm-svn: 372938