This allows us to combine multiple Errors together without repeating the prefix
information. Also fixes several cases where two "COMPILATION ERROR" prefixes
would be prepended to the same message when errors with prefixes and locations
were produced by the lexer and parser.
My intent is to add CARBON_ bas a prefix, and this makes that easier by creating a tool for auto-fixing guards in general.
string_literal is a manual fix -- it had no guard and I didn't automate that (technically I think I could, especially by enforcing the file header/footer, but it didn't feel quite worth it to me).