[Remarks] Add support for linking remarks

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