Added API:
SourceLocations.ByPath
SourceLocations.ByDescriptor
SourceLocation.Next
SourcePath.String
SourcePath.Equal
We modify compiler/protogen to use SourceLocations.ByDescriptor.
In retrospect, if this had existed during the development of protogen,
we would not have exposed protogen.Location and related fields.
Change-Id: I58f17e59f90b9ba16f0982c4b71c2542e4ff6e75
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/238000
Reviewed-by: Damien Neil <dneil@google.com>