Those macros may be defined, or not. We should support both cases, so use std::string as it also defines the length of the string.

ShaderGen: Remove virtual methods and string from ShaderGeneratorInterface.

Also gets rid of global headers

This fixes the crashes occuring at startup with a non-empty shader cache.
Because LinearDiskCache reads/writes to the storage of ShaderUid, ShaderUid must be trivially copyable.
Additionally, adds a static assert to LinearDiskCache to ensure this doesn't happen in the future.

The initialization of ShaderUid data has been moved to the code generation functions, so the above condition holds true.