[objcmt] When emitting a remap file, use a json format with the edit entries,...

[objcmt] When emitting a remap file, use a json format with the edit entries, instead of applying the changes
to a temp file directly.

This allows to combine the edits when they can be different based on whether you saw
the implementation or not, e.g. with the designated initializer migration.

llvm-svn: 197076