Allow the last member of a struct to be an unsized/zero-length array.

This enables the C truck of allocating a dynamic amount of storage for
the struct in order to extend out the array to the desired length.

tests/struct-zero-len-array-member.ispc

@@ -0,0 +1,24 @@
+
+struct Foo {
+    float x;
+    float a[0];
+};
+
+export uniform int width() { return programCount; }
+
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    uniform int nFloats = 3+programCount;
+    varying Foo * uniform ptr = (varying Foo * uniform)(uniform new varying int32[nFloats]);
+    memset(ptr, 0, nFloats*sizeof(int32));
+    
+    for (uniform int i = 0; i < nFloats-1; ++i)
+        ptr->a[i] = i;
+    ptr->x = aFOO[programIndex]; 
+
+    RET[programIndex] = ptr->a[1+programIndex]; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1 + programIndex;
+}