stage2: implement cmpxchg and improve comptime eval

* Implement Sema for `@cmpxchgWeak` and `@cmpxchgStrong`. Both runtime
   and comptime codepaths are implement.
 * Implement Codegen for LLVM backend and C backend.
 * Add LazySrcLoc.node_offset_builtin_call_argX 3...5
 * Sema: rework comptime control flow.
   - `error.ComptimeReturn` is used to signal that a comptime function
     call has returned a result (stored in the Inlining struct).
     `analyzeCall` notices this and handles the result.
   - The ZIR instructions `break_inline`, `block_inline`,
     `condbr_inline` are now redundant and can be deleted. `break`,
     `block`, and `condbr` function equivalently inside a comptime scope.
   - The ZIR instructions `loop` and `repeat` also are modified to
     directly perform comptime control flow inside a comptime scope,
     skipping an unnecessary mechanism for analysis of runtime code.
     This makes Zig perform closer to an interpreter when evaluating
     comptime code.
 * Sema: zirRetErrValue looks at Sema.ret_fn_ty rather than sema.func
   for adding to the inferred error set. This fixes a bug for
    inlined/comptime function calls.
 * Implement ZIR printing for cmpxchg.
 * stage1: make cmpxchg respect --single-threaded
   - Our LLVM C++ API wrapper failed to expose this boolean flag before.
 * Fix AIR printing for struct fields showing incorrect liveness data.

src/stage1/codegen.cpp

@@ -5723,7 +5723,7 @@ static LLVMValueRef ir_render_cmpxchg(CodeGen *g, Stage1Air *executable, Stage1A
     LLVMAtomicOrdering failure_order = to_LLVMAtomicOrdering(instruction->failure_order);
 
     LLVMValueRef result_val = ZigLLVMBuildCmpXchg(g->builder, ptr_val, cmp_val, new_val,
-            success_order, failure_order, instruction->is_weak);
+            success_order, failure_order, instruction->is_weak, g->is_single_threaded);
 
     ZigType *optional_type = instruction->base.value->type;
     assert(optional_type->id == ZigTypeIdOptional);