Sema: fix UAF in zirClosureGet

Previously if a decl failed its capture scope would be deallocated and
set to undefined which would then lead to invalid dereference in
`zirClosureGet`. To avoid this set the capture scope to a special
failed state and fail the current decl with dependency failure if
the failed state is encountered in `zirClosureGet`.

Closes #12433
Closes #12530
Closes #12593

test/cases/compile_errors/closure_get_depends_on_failed_decl.zig

@@ -0,0 +1,26 @@
+pub inline fn instanceRequestAdapter() void {}
+
+pub inline fn requestAdapter(
+    comptime callbackArg: fn () callconv(.Inline) void,
+) void {
+    _ = (struct {
+        pub fn callback() callconv(.C) void {
+            callbackArg();
+        }
+    }).callback;
+    instanceRequestAdapter(undefined); // note wrong number of arguments here
+}
+
+inline fn foo() void {}
+
+pub export fn entry() void {
+    requestAdapter(foo);
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :11:5: error: expected 0 argument(s), found 1
+// :1:12: note: function declared here
+// :17:19: note: called from here

test/cases/compile_errors/closure_get_in_param_ty_instantiate_incorrectly.zig

@@ -0,0 +1,24 @@
+fn Observable(comptime T: type) type {
+    return struct {
+        fn map(Src: T, Dst: anytype, function: fn (T) Dst) Dst {
+            _ = Src;
+            _ = function;
+            return Observable(Dst);
+        }
+    };
+}
+
+fn u32Tou64(x: u32) u64 {
+    _ = x;
+    return 0;
+}
+
+pub export fn entry() void {
+    Observable(u32).map(u32, u64, u32Tou64(0));
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :17:25: error: expected type 'u32', found 'type'