Fix some issues with signedness in preprocessor expressions. Do also disallow

comma expressions since the aren't compliant and collide with macro invocations.

test/err/bug2523.c

@@ -0,0 +1,3 @@
+#if (1, 0) < 0
+#error
+#endif

test/val/bug2523.c

@@ -0,0 +1,29 @@
+#if (0u - 1) < 0
+#error
+#endif
+
+#if !1u - 1 > 0
+#error
+#endif
+
+#if (1 & 1u) - 2 < 0
+#error
+#endif
+
+#if (1 | 1u) - 2 < 0
+#error
+#endif
+
+#if (1 ^ 1u) - 2 < 0
+#error
+#endif
+
+#if (1u >> 1) - 2 < 0
+#error
+#endif
+
+#if (0u << 1) - 1 < 0
+#error
+#endif
+
+int main() { return 0; }