We changed to `llvm.compiler.used` because of the behaviour of `gold`,
which refuses to coalesce sections that have different `SHF_GNU_RETAIN`
flags, which causes problems with metadata.
Originally I thought we were going to have to generate two sections
with distinct names and have the runtime look for both of them, but
it turns out that the runtime only wants to see sections that have
`SHF_GNU_RETAIN` in any case. It's really the reflection code that
is interested in being able to see non-retained sections. The upshot
is that we don't need to use `llvm.compiler.used`; it's just fine if
we have duplicate sections, as long as the reflection code looks for
them when it's inspecting an ELF image.
This also means we no longer need to pass `-z nostart-stop-gc` to the
linker if we're using `lld`.
rdar://123504095
Foundation imports CoreFoundation with `@_implementationOnly`,
so CoreFoundation's modulemap won't be read, and the dependent libraries
of CoreFoundation will not be automatically linked when using static
linking.
For example, CoreFoundation depends on libicui18n and it's modulemap has
`link "icui18n"` statement. If Foundation imports CoreFoundation with
`@_implementationOnly` as a private dependency, the toolchain doesn't have
CoreFoundation's modulemap and Foundation's swiftmodule doesn't import
CoreFoundation. So the swiftc can't know that libicui18n is required.
This new option will add LINK_LIBRARY entry in swiftmodule to
specify dependent libraries (in the example case, Foundation's
swiftmodule should have LINK_LIBRARY entry of libicui18n)
See also: [Autolinking behavior of @_implementationOnly with static linking](https://forums.swift.org/t/autolinking-behavior-of-implementationonly-with-static-linking/44393)
The layouts of resilient value types shipped in the Swift 5 standard library
x and overlays will forever be frozen in time for backward deployment to old
Objective-C runtimes. This PR ensures that even if the layouts of these types
evolve in the future, binaries built to run on the old runtime will continue
to lay out class instances in a manner compatible with Swift 5.
Fixes <rdar://problem/45646886>.
