[shims] Remove the _SwiftDispatchOverlayShims module

[lorentey]

This module used to be a private implementation detail of the Dispatch overlay on Apple platforms, and swift-corelibs-libdispatch elsewhere.

Darwin hasn't been using this module for a couple years now, and swift-corelibs-dispatch stopped using it with swiftlang/swift-corelibs-libdispatch#592. We are now free to remove it from the Swift toolchain -- which is good, as it contains declarations that are specific to Dispatch, and the compiler/stdlib is not the right place to maintain them.

[lorentey]

@swift-ci test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 4e90871

[lorentey]

16:59:02 
java.io.EOFException
16:59:02 	at java.io.ObjectInputStream$PeekInputStream.readFully(ObjectInputStream.java:2798)
16:59:02 	at java.io.ObjectInputStream$BlockDataInputStream.readShort(ObjectInputStream.java:3273)
16:59:02 	at java.io.ObjectInputStream.readStreamHeader(ObjectInputStream.java:933)
16:59:02 	at java.io.ObjectInputStream.<init>(ObjectInputStream.java:395)
16:59:02 	at hudson.remoting.ObjectInputStreamEx.<init>(ObjectInputStreamEx.java:49)
16:59:02 	at hudson.remoting.Command.readFrom(Command.java:142)
16:59:02 	at hudson.remoting.Command.readFrom(Command.java:128)
16:59:02 	at hudson.remoting.AbstractSynchronousByteArrayCommandTransport.read(AbstractSynchronousByteArrayCommandTransport.java:35)
16:59:02 	at hudson.remoting.SynchronousCommandTransport$ReaderThread.run(SynchronousCommandTransport.java:61)
16:59:02 Caused: java.io.IOException: Unexpected termination of the channel
16:59:02 	at hudson.remoting.SynchronousCommandTransport$ReaderThread.run(SynchronousCommandTransport.java:75)
16:59:02 Caused: java.io.IOException: Backing channel 'macOS-55' is disconnected.

[lorentey]

@swift-ci test macOS platform

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 4e90871

[lorentey]

Ah, this is unrelated & will be fixed by #41298:

<unknown>:0: warning: using sysroot for 'iPhoneSimulator' but targeting 'MacOSX'
<unknown>:0: warning: using sysroot for 'iPhoneSimulator' but targeting 'MacOSX'
<unknown>:0: warning: module interfaces are only supported with Swift language version 5 or later (currently using -swift-version 4)
<unknown>:0: error: module 'Test' was created for incompatible target x86_64-apple-macosx11.0: /Users/buildnode/jenkins/workspace/swift-PR-macos/branch-main/buildbot_incremental/swift-macosx-x86_64/test-iphonesimulator-x86_64/ModuleInterface/Output/back-deploy-attr.swift.tmp/Test.swiftmodule

[lorentey]

@swift-ci smoke test macOS platform