Sema: add @errorCast which works for both error sets and error unions

Closes ziglang#17343

Author: Vexu

compile_errors/explicit_error_set_cast_known_at_comptime_violates_error_sets.zig

@@ -2,7 +2,7 @@ const Set1 = error{ A, B };
 const Set2 = error{ A, C };
 comptime {
     var x = Set1.B;
-    var y: Set2 = @errSetCast(x);
+    var y: Set2 = @errorCast(x);
     _ = y;
 }
 

compile_errors/int_to_err_non_global_invalid_number.zig

@@ -8,7 +8,7 @@ const Set2 = error{
 };
 comptime {
     var x = @intFromError(Set1.B);
-    var y: Set2 = @errSetCast(@errorFromInt(x));
+    var y: Set2 = @errorCast(@errorFromInt(x));
     _ = y;
 }
 

safety/@errSetCast error not present in destination.zig safety/@errorCast error not present in destination.zig

@@ -14,7 +14,7 @@ pub fn main() !void {
     return error.TestFailed;
 }
 fn foo(set1: Set1) Set2 {
-    return @errSetCast(set1);
+    return @errorCast(set1);
 }
 // run
 // backend=llvm