Overly agressive constness check on comptime copied slices

[N00byEdge]

  comptime var x: []u8 = &[_]u8{};
  x = x ++ "a";

Fails with the following error:

./example.zig:3:9: error: expected type '[]u8', found '*const [1:0]u8'
  x = x ++ "a";
        ^

Godbolt: https://gcc.godbolt.org/z/he94PY3bx
Should probably be allowed since this is a value copy, not a reference

[N00byEdge]

Workaround:

  comptime var x: []const u8 = &[_]u8{};
  x = x ++ "a";