Parse and then mostly ignore "signed" qualifier.

Just issue errors if both "signed" and "unsigned" are specified,
or if "signed" is applied to a non-int type.

tests_errors/signed-unsigned.ispc

@@ -0,0 +1,5 @@
+// Error: Illegal to apply both "signed" and "unsigned" qualifiers
+
+int foo() {
+    signed unsigned int x;
+}