Test only passes for signed char

The original Quantifiers-type2 test fails on aarch64 as it has unsigned
char. Explicitly use signed char in this test and create a KNOWNBUG test
for unsigned char.

Author: tautschnig

regression/cbmc/Quantifiers-type2/main.c

@@ -5,7 +5,7 @@ int main()
 
   // clang-format off
   // clang-format would rewrite the "==>" as "== >"
-  __CPROVER_assume( __CPROVER_forall { char i; (i>=0 && i<2) ==> b[i]>=10 && b[i]<=10 } );
+  __CPROVER_assume( __CPROVER_forall { signed char i; (i>=0 && i<2) ==> b[i]>=10 && b[i]<=10 } );
   __CPROVER_assume( __CPROVER_forall { unsigned i; (i>=0 && i<2) ==> c[i]>=10 && c[i]<=10 } );
   // clang-format on
 

regression/cbmc/Quantifiers-type2/unsigned-char.c

@@ -0,0 +1,16 @@
+int main()
+{
+  int b[2];
+  int c[2];
+
+  // clang-format off
+  // clang-format would rewrite the "==>" as "== >"
+  __CPROVER_assume( __CPROVER_forall { unsigned char i; (i>=0 && i<2) ==> b[i]>=10 && b[i]<=10 } );
+  __CPROVER_assume( __CPROVER_forall { unsigned i; (i>=0 && i<2) ==> c[i]>=10 && c[i]<=10 } );
+  // clang-format on
+
+  assert(b[0] == 10 && b[1] == 10);
+  assert(c[0] == 10 && c[1] == 10);
+
+  return 0;
+}

regression/cbmc/Quantifiers-type2/unsigned-char.desc

@@ -0,0 +1,16 @@
+CORE broken-cprover-smt-backend no-new-smt
+unsigned-char.c
+
+^\*\* Results:$
+^\[main.assertion.1\] line 12 assertion b\[.*0\] == 10 && b\[.*1\] == 10: SUCCESS$
+^\[main.assertion.2\] line 13 assertion c\[.*0\] == 10 && c\[.*1\] == 10: SUCCESS$
+^VERIFICATION SUCCESSFUL$
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+--
+skip_typecast as used in expr_eq of boolbv_quantifier.cpp no longer applies when
+using an unsigned char, which makes our quantifier instantiation fail when
+triggered from our in-tree SMT solver. We need to audit all uses of
+skip_typecast as some of these may even be unsound.