[ELF] Support ELFCOMPRESS_ZSTD input

[MaskRay]

Summary

so that lld accepts relocatable object files produced by clang -c -g -gz=zstd.

We don't want to increase the size of InputSection, so do redundant but cheap
ch_type checks instead.

[MaskRay]

rebase

[MaskRay]

Fix gdb-index

[MaskRay]

add release note

[MaskRay]

simplify test

[dblaikie]

Looks like this didn't get reviewed?

[MaskRay]

In D129406#3780891, @dblaikie wrote:

Looks like this didn't get reviewed?

Happy to address changes if @ikudrin or @peter.smith has any suggestion.
The changes are pretty mechanical and a dependent was accepted.

[dblaikie]

In D129406#3780896, @MaskRay wrote:

In D129406#3780891, @dblaikie wrote:

Looks like this didn't get reviewed?

Happy to address changes if @ikudrin or @peter.smith has any suggestion.
The changes are pretty mechanical and a dependent was accepted.

The LLVM review process is that if something's sent for review it should not be committed until that review is completed ( https://llvm.org/docs/CodeReview.html#code-review-workflow "Specifically, once a patch is sent out for review, it needs an explicit approval before it is committed. Do not assume silent approval, or solicit objections to a patch with a deadline." ) . (we don't want to encourage people to send things for review that they believe needs review, then commit due to lack of feedback in the time they'd like to receive it)