Shrink most LOG callsites by one instruction (SysV x86_64 ABI) by dispatching to per-severity constructors and omitting the severity argument.

In that particular ABI, the empty tag structs are NO_CLASS and don't appear in the generated code.

https://godbolt.org/z/crqaPh8Kv

PiperOrigin-RevId: 527989179
Change-Id: I365f11f6ca1de2623690d0a04e6f02f7de82b1fc