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/unsigned-float-1.ispc

@@ -0,0 +1,5 @@
+// Can't apply "unsigned" qualifier to "float" type
+
+struct Foo {
+    unsigned float x;
+};