Use line comments instead of block comment for generated license headers
This makes it shorter. With the spdx license header in !225 (merged) the headers are generally 2 or 3 lines long, so we don't need to make it 50% longer by closing and opening the comment blog.
before with !225 (merged)
/*
* SPDX-File....
* SPDX-Licen....
*/
after:
// SPDX-File...
// SPDX-License...