Re-Add Synchronization Import on non-Darwin

[jmschonfeld]

Previously, we had issues with import Synchronization in the toolchain - this was due to the rpaths of the produced libraries. We previously applied -no-toolchain-stdlib-rpath so that the rpath of the libraries in the build folder had no rpaths set. When the libraries are installed into the toolchain, $ORIGIN was appended which is correct for the installed library. However, XCTest's tests link against these libraries directly in the build folder before they are installed. Loading the stdlib worked because XCTest's tests also link the stdlib and so the library was found in the cache, however since XCTest's tests do not link Synchronization it wasn't found in the cache and since the rpaths list was empty the library couldn't be found.

Instead, we now remove the -no-toolchain-stdlib-rpath argument so that the libraries in the build directory have rpaths pointing to the swift build folder. However, we also now set INSTALL_REMOVE_ENVIRONMENT_RPATH to ON so that when copying to the installation folder, this path is replaced by $ORIGIN instead of just appending the $ORIGIN path to the list so that the produced toolchain doesn't contain local rpaths.

This should also resolve a related warning in the macro due to the lack of rpaths while loading the macro from the build folder to build FoundationEssentials itself:

/home/build-user/swift-foundation/Sources/FoundationEssentials/Predicate/Expression.swift:32:14: warning: external macro implementation type 'FoundationMacros.ExpressionMacro' could not be found for macro 'Expression'; failed to load library plugin '/home/build-user/build/buildbot_linux/foundation-linux-x86_64/lib/libFoundationMacros.so' in plugin server '/home/build-user/build/buildbot_linux/swift-linux-x86_64/lib/swift/host/libSwiftInProcPluginServer.so'; loader error: libswift_StringProcessing.so: cannot open shared object file: No such file or directory

[jmschonfeld]

@swift-ci please test

[jmschonfeld]

Full toolchain tests and a full toolchain build is running at swiftlang/swift#75321

[parkera]

Awesome, thanks so much for tracking this down.

[jmschonfeld]

@swift-ci please test

[Azoy]

@swift-ci please test

[jmschonfeld]

@swift-ci please test

[parkera]

For now I had to remove the file that used Synchronization, so this PR actually becomes somewhat of a no-op in the short term. However, I am sure we will want Synchronization back soon.

[jmschonfeld]

For now I had to remove the file that used Synchronization, so this PR actually becomes somewhat of a no-op in the short term. However, I am sure we will want Synchronization back soon.

Sounds good - I've rebased on main so that we can still land this so that PredicateExpressions.Variable can still use Synchronization (and wherever else we need it in the future)

[jmschonfeld]

@swift-ci please test