This is how it should've been and brings it more in line with
std::string_view. There should be no functional change here.
This is mostly mechanical from a custom clang-tidy check, with a lot of
manual fixups. It uncovers a lot of minor inefficiencies.
This doesn't actually modify StringRef yet, I'll do that in a follow-up.
Remarks are usually emitted per-TU, and for generating a standalone
remark file that can be shipped with the linked binary we need some kind
of tool to merge everything together.
The remarks::RemarkLinker class takes care of this and:
* Deduplicates remarks
* Filters remarks with no debug location
* Merges string tables from all the entries
As an output, it provides an iterator range that can be used to
serialize the remarks to a file.
Differential Revision: https://reviews.llvm.org/D69141
