x/tools/go/gcexportdata: support export using indexed format

[alandonovan]

We need to port the reader of indexed import data ($GOROOT/src/cmd/compile/internal/gc/iexport.go) to produce go/types data structures, analogous to what we do for the $$B format, which is deprecated.

@stamblerre

[stamblerre]

It seems like gcexportdata.Read is already reading indexed export data as of golang/tools@9a70f1f.

@alandonovan or @griesemer: is there anything else that needs to be done here?

[alandonovan]

I corrected the issue title to mean "write", not "read".

[stamblerre]

Thanks - I'll start working on this now.

[gopherbot]

Change https://golang.org/cl/156901 mentions this issue: go/internal/gcimporter: write export data for go/types

[griesemer]

@stamblerre Is this finished, i.e., can we close this issue?

[stamblerre]

I think the final step here would be to have the gcexportdata.Write function actually write the indexed export data format (https://github.com/golang/tools/blob/master/go/gcexportdata/gcexportdata.go#L102). I will send out a CL to that effect ASAP.