add two failing tests for preincrement on _Bool

This adds two tests that expose bugs when doing preincrement on C99 _Bool.

Author: kroening

regression/cbmc/Bool/bool6.c

@@ -0,0 +1,19 @@
+int main()
+{
+  _Bool u = 1;
+  _Bool *p = &u;
+
+  // combination of _Bool, dereference, and pre-increment side-effect
+  if(++(*p) != 1)
+    __CPROVER_assert(0, "");
+
+  // combination of _Bool, dereference, and compound assignment side-effect
+  if(((*p) += 1) != 1)
+    __CPROVER_assert(0, "");
+
+  // combination of _Bool, dereference, and assignment side-effect
+  if(((*p) = 1) != 1)
+    __CPROVER_assert(0, "");
+
+  __CPROVER_assert(*p == 1, "");
+}

regression/cbmc/Bool/bool6.desc

@@ -0,0 +1,8 @@
+KNOWNBUG
+bool6.c
+
+^EXIT=0$
+^SIGNAL=0$
+^VERIFICATION SUCCESSFUL$
+--
+^warning: ignoring

regression/cbmc/Bool/bool7.c

@@ -0,0 +1,11 @@
+#include <assert.h>
+
+int main()
+{
+  _Bool u = 1;
+
+  // assert the type of side effects on _Bool
+  assert(sizeof(++u) == sizeof(_Bool));
+  assert(sizeof(u += 1) == sizeof(_Bool));
+  assert(sizeof(u = 1) == sizeof(_Bool));
+}

regression/cbmc/Bool/bool7.desc

@@ -0,0 +1,8 @@
+KNOWNBUG
+bool7.c
+
+^EXIT=0$
+^SIGNAL=0$
+^VERIFICATION SUCCESSFUL$
+--
+^warning: ignoring