- FileLineInfoSpecifier::Default isn't the default for anything.
Rename to RawValue, which accurately reflects its role.
- Most functions that take a part of a FileLineInfoSpecifier end up
constructing a full one later or plumb two values through. Make them
all just take a complete FileLineInfoSpecifier.
- Printing basenames only was handled differently from all other
variants, make it parallel to all the other variants.