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-struct.ispc

@@ -0,0 +1,7 @@
+// "signed" qualifier is illegal with non-integer type "uniform struct Foo"
+
+struct Foo {
+    float x;
+};
+
+signed Foo f;