SPV arrays: Add support for runtime-sized array types and arrays of arrays.

This includes run-time block.member.length() (OpArrayLength).

SPIRV/SpvBuilder.h

@@ -102,6 +102,7 @@ public:
     Id makeVectorType(Id component, int size);
     Id makeMatrixType(Id component, int cols, int rows);
     Id makeArrayType(Id element, unsigned size);
+    Id makeRuntimeArray(Id element);
     Id makeFunctionType(Id returnType, std::vector<Id>& paramTypes);
     Id makeImageType(Id sampledType, Dim, bool depth, bool arrayed, bool ms, unsigned sampled, ImageFormat format);
     Id makeSampledImageType(Id imageType);