shader/shader_ir: Make Comment() take a std::string by value

This allows for forming comment nodes without making unnecessary copies
of the std::string instance.

e.g. previously:

Comment(fmt::format("Base address is c[0x{:x}][0x{:x}]",
        cbuf->GetIndex(), cbuf_offset));

Would result in a copy of the string being created, as CommentNode()
takes a std::string by value (a const ref passed to a value parameter
results in a copy).

Now, only one instance of the string is ever moved around. (fmt::format
returns a std::string, and since it's returned from a function by value,
this is a prvalue (which can be treated like an rvalue), so it's moved
into Comment's string parameter), we then move it into the CommentNode
constructor, which then moves the string into its member variable).