[dsymutil] Add support for linking remarks

This adds support to dsymutil for linking remark files and placing them
in the final .dSYM bundle.

The result will be placed in:

* a.out.dSYM/Contents/Resources/Remarks/a.out
or
* a.out.dSYM/Contents/Resources/Remarks/a.out-<arch> for universal binaries

When multi-threaded, this runs a third thread which loops over all the
object files and parses remarks as it finds __remarks sections.

Testing this involves running dsymutil on pre-built binaries and object
files, then running llvm-bcanalyzer on the final result to check for
remarks.

Differential Revision: https://reviews.llvm.org/D69142