Fix bug with taking references of temporaries.

Previously, the compiler would crash if e.g. the program passed a
temporary value to a function taking a const reference.  This change
fixes ReferenceExpr::GetValue() to handle this case and allocate
temporary storage for the temporary so that the pointer to that
storage can be used for the reference value.

tests/ref-as-temporary.ispc

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