Update function overload resolution logic.

Closer compatibility with C++: given a non-reference type, treat matching
to a non-const reference of that type as a better match than a const
reference of that type (rather than both being equal cost).

Issue #224.

tests/func-overload-refs.ispc

@@ -0,0 +1,14 @@
+
+export uniform int width() { return programCount; }
+
+float foo(float &a) { return 1; }
+float foo(const float &a) { return 2; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    float x = 0;
+    RET[programIndex] = foo(x); 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1;
+}