Since strings don't have a constexpr constructor, these won't have any
effect anyways. Furthermore, this is explicitly disallowed by the
standard, and makes Clang tools freak out.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
