Add test that plain int bitfields are unsigned

We want to make sure this doesn't change when #1095 is fixed;
unsigned is much more efficient.

test/val/plain-int-bitfield.c

@@ -0,0 +1,63 @@
+/*
+  Copyright 2020 Google LLC
+
+  This software is provided 'as-is', without any express or implied
+  warranty. In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+/*
+  Tests that plain int bit-fields are unsigned.
+*/
+
+#include <stdio.h>
+
+static unsigned char failures = 0;
+
+static struct plain_ints {
+    int x : 4;
+    int y : 10;
+} pi = {15, 700};
+
+static void test_plain_int_bitfields (void)
+{
+    if (pi.x != 15) {
+        printf ("Got pi.x = %u, expected 15.\n", pi.x);
+        failures++;
+    }
+    if (pi.y != 700) {
+        printf ("Got pi.y = %u, expected 700.\n", pi.y);
+        failures++;
+    }
+
+    pi.x = 3;
+    pi.y = 1023;
+
+    if (pi.x != 3) {
+        printf ("Got pi.x = %u, expected 3.\n", pi.x);
+        failures++;
+    }
+    if (pi.y != 1023) {
+        printf ("Got pi.y = %u, expected 1023.\n", pi.y);
+        failures++;
+    }
+}
+
+int main (void)
+{
+    test_plain_int_bitfields ();
+    printf ("failures: %u\n", failures);
+    return failures;
+}