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-float-1.ispc

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

tests_errors/signed-float.ispc

@@ -0,0 +1,5 @@
+// "signed" qualifier is illegal with non-integer type "float"
+
+int foo() {
+    signed float x;
+}

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;

tests_errors/signed-unsigned.ispc

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

tests_errors/unsigned-float-1.ispc

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

tests_errors/unsigned-struct.ispc

@@ -0,0 +1,7 @@
+// "unsigned" qualifier is illegal with "uniform struct Foo" typ
+
+struct Foo {
+    float x;
+};
+
+unsigned Foo f;