Sema: dummy implementation of @errorReturnTrace

Also update std/build.zig to use stage2 function pointer semantics.

This gets us a little bit closer to `zig build` working, although it is
now hitting a new crash in the compiler.

lib/std/build.zig

@@ -3256,11 +3256,16 @@ const ThisModule = @This();
 pub const Step = struct {
     id: Id,
     name: []const u8,
-    makeFn: fn (self: *Step) anyerror!void,
+    makeFn: MakeFn,
     dependencies: ArrayList(*Step),
     loop_flag: bool,
     done_flag: bool,
 
+    const MakeFn = switch (builtin.zig_backend) {
+        .stage1 => fn (self: *Step) anyerror!void,
+        else => *const fn (self: *Step) anyerror!void,
+    };
+
     pub const Id = enum {
         top_level,
         lib_exe_obj,
@@ -3279,7 +3284,7 @@ pub const Step = struct {
         custom,
     };
 
-    pub fn init(id: Id, name: []const u8, allocator: Allocator, makeFn: fn (*Step) anyerror!void) Step {
+    pub fn init(id: Id, name: []const u8, allocator: Allocator, makeFn: MakeFn) Step {
         return Step{
             .id = id,
             .name = allocator.dupe(u8, name) catch unreachable,

lib/std/start.zig

@@ -408,11 +408,6 @@ fn posixCallMainAndExit() noreturn {
         // Initialize the TLS area.
         std.os.linux.tls.initStaticTLS(phdrs);
 
-        if (builtin.zig_backend == .stage2_llvm) {
-            root.main();
-            exit2(0);
-        }
-
         // The way Linux executables represent stack size is via the PT_GNU_STACK
         // program header. However the kernel does not recognize it; it always gives 8 MiB.
         // Here we look for the stack size in our program headers and use setrlimit
@@ -454,6 +449,10 @@ fn callMainWithArgs(argc: usize, argv: [*][*:0]u8, envp: [][*:0]u8) u8 {
     std.os.argv = argv[0..argc];
     std.os.environ = envp;
 
+    if (builtin.zig_backend == .stage2_llvm) {
+        return @call(.{ .modifier = .always_inline }, callMain, .{});
+    }
+
     std.debug.maybeEnableSegfaultHandler();
 
     return initEventLoopAndCallMain();

src/Sema.zig

@@ -12439,7 +12439,11 @@ fn zirErrorReturnTrace(
     extended: Zir.Inst.Extended.InstData,
 ) CompileError!Air.Inst.Ref {
     const src: LazySrcLoc = .{ .node_offset = @bitCast(i32, extended.operand) };
-    return sema.fail(block, src, "TODO: Sema.zirErrorReturnTrace", .{});
+    const unresolved_stack_trace_ty = try sema.getBuiltinType(block, src, "StackTrace");
+    const stack_trace_ty = try sema.resolveTypeFields(block, src, unresolved_stack_trace_ty);
+    const opt_stack_trace_ty = try Type.optional(sema.arena, stack_trace_ty);
+    // https://github.com/ziglang/zig/issues/11259
+    return sema.addConstant(opt_stack_trace_ty, Value.@"null");
 }
 
 fn zirFrame(