Changes

[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