spirv: make default generic address space for vulkan Function

We are not using Private variables. This needs to be cleaned up a bit
more, this will happen with the general address space improvements.

src/codegen/spirv.zig

@@ -1882,7 +1882,7 @@ const NavGen = struct {
                 else => unreachable,
             },
             .shared => .Workgroup,
-            .local => .Private,
+            .local => .Function,
             .global => switch (target.os.tag) {
                 .opencl => .CrossWorkgroup,
                 .vulkan => .PhysicalStorageBuffer,