AstGen: add error for invalid string comparisons

These operations are allowed because the string literals are just
pointers but they produce unexpected results. These errors prevent
beginners from shooting themselves in the foot while still allowing
advanced users to circumvent them if they desire to do so.

Closes #8290

test/cases/compile_errors/invalid_compare_string.zig

@@ -0,0 +1,29 @@
+comptime {
+    var a = "foo";
+    if (a == "foo") unreachable;
+}
+comptime {
+    var a = "foo";
+    if (a == ("foo")) unreachable; // intentionally allow
+}
+comptime {
+    var a = "foo";
+    switch (a) {
+        "foo" => unreachable,
+        else => {},
+    }
+}
+comptime {
+    var a = "foo";
+    switch (a) {
+        ("foo") => unreachable, // intentionally allow
+        else => {},
+    }
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :3:11: error: cannot compare strings with ==
+// :12:9: error: cannot switch on strings