[msan] add a regression test for PR32842

[glider]

Summary

Make sure MSan doesn't miss a bug comparing two integers with defined low bits.

[glider]

Landed in r302788.

test/msan/pr32842.c

+// Regression test for https://bugs.llvm.org/show_bug.cgi?id=32842
+//
+// RUN: %clang_msan -g %s -o %t
+// RUN: not %run %t 2>&1 | FileCheck  %s
+
+struct iphdr {
+  unsigned char pad1: 2, ihl:4, pad2: 2;
+};
+
+int raw_send_hdrinc(unsigned long int length) {
+  struct iphdr iph;
+  if (iph.ihl * 4 > length) {
+    return 1;
+  }
+  return 0;
+}
+
+int main(int argc, char *argv[]) {
+  return raw_send_hdrinc(12);
+}
+
+// CHECK: WARNING: MemorySanitizer: use-of-uninitialized-value