llvm-wrapper: add alignment argument to CreateAtomicCmpXchg

As far as I can tell what we've been getting is llvm::MaybeAlign(), so
just use that for now. This is required sometime after
24539f1.

Author: durin42

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -382,9 +382,15 @@ LLVMRustBuildAtomicCmpXchg(LLVMBuilderRef B, LLVMValueRef Target,
                            LLVMValueRef Old, LLVMValueRef Source,
                            LLVMAtomicOrdering Order,
                            LLVMAtomicOrdering FailureOrder, LLVMBool Weak) {
+#if LLVM_VERSION_GE(12,0)
+  AtomicCmpXchgInst *ACXI = unwrap(B)->CreateAtomicCmpXchg(
+      unwrap(Target), unwrap(Old), unwrap(Source), llvm::MaybeAlign(), fromRust(Order),
+      fromRust(FailureOrder));
+#else
   AtomicCmpXchgInst *ACXI = unwrap(B)->CreateAtomicCmpXchg(
       unwrap(Target), unwrap(Old), unwrap(Source), fromRust(Order),
       fromRust(FailureOrder));
+#endif
   ACXI->setWeak(Weak);
   return wrap(ACXI);
 }