Cleanup the plumbing for DILineInfoSpecifier. [NFC]

Summary:
1. FileLineInfoSpecifier::Default isn't the default for anything.
Rename to RawValue, which accurately reflects its role.
2. 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.
3. Printing basenames only was handled differently from all other
variants, make it parallel to all the other variants.

Reviewers: jhenderson

Subscribers: hiraditya, MaskRay, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76394