spirv: don't emit invalid debuginfo for buffer references

Currently no debug info is emitted for buffer reference types, which
resulted in the SPIR-V backend code asserting when trying to emit the
debug info for struct member that had such a type. Instead, the code now
skips such struct members. Full debug info for buffer references may
require forward references in the debug info instructions, which is
currently prohibited by the spec.

Test/spv.debuginfo.bufferref.glsl.frag

@@ -0,0 +1,28 @@
+#version 450 core
+#extension GL_EXT_buffer_reference : enable
+
+layout(buffer_reference, std430) buffer MeshVertexPositions {
+  float data[];
+};
+
+struct Mesh {
+  MeshVertexPositions positions;
+};
+
+layout(set = 0, binding = 0) readonly buffer PerPass_meshes {
+  Mesh data[];
+} perPass_meshes;
+
+layout(location = 0) out vec4 out_fragColor;
+
+layout(location = 0) in flat uint tri_idx0;
+
+void main() {
+    Mesh meshData = perPass_meshes.data[tri_idx0];
+
+    vec3 vertex_pos0 = vec3(meshData.positions.data[3 * tri_idx0],
+                            meshData.positions.data[3 * tri_idx0 + 1],
+                            meshData.positions.data[3 * tri_idx0 + 2]);
+    
+    out_fragColor = vec4(vertex_pos0, 1.0);
+}