[AutoDiff] fix _Differentiation rpath

[marcrasi]

I'm not very confident that this is the right thing to do. @compnerd could you take a look and let me know what you think?

The problem that this is fixing:

# In a SwiftPM package with a single main file:
$ cat Sources/testpackage/main.swift
import _Differentiation
print(gradient(at: Float(6), in: { $0 * $0 }))
$ swift run
dyld: Library not loaded: /usr/lib/swift/libswift_Differentiation.dylib
  Referenced from: /Users/marcrasi/testpackage/.build/x86_64-apple-macosx/debug/testpackage
  Reason: image not found
Abort trap: 6

Some commands that I ran for debugging:

$ otool -L /Users/marcrasi/testpackage/.build/x86_64-apple-macosx/debug/testpackage
/Users/marcrasi/testpackage/.build/x86_64-apple-macosx/debug/testpackage:
	/System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation (compatibility version 150.0.0, current version 1675.129.0)
	/usr/lib/libobjc.A.dylib (compatibility version 1.0.0, current version 228.0.0)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1281.100.1)
	/usr/lib/swift/libswift_Differentiation.dylib (compatibility version 1.0.0, current version 0.0.0)
	@rpath/libswiftCore.dylib (compatibility version 1.0.0, current version 0.0.0)
	@rpath/libswiftDarwin.dylib (compatibility version 1.0.0, current version 0.0.0, weak)
	@rpath/libswiftSwiftOnoneSupport.dylib (compatibility version 1.0.0, current version 0.0.0)

$ otool -l /Users/marcrasi/testpackage/.build/x86_64-apple-macosx/debug/testpackage | grep RPATH -A2
          cmd LC_RPATH
      cmdsize 32
         path /usr/lib/swift (offset 12)
--
          cmd LC_RPATH
      cmdsize 32
         path @loader_path (offset 12)
--
          cmd LC_RPATH
      cmdsize 192
         path /Users/marcrasi/swift-base/build/Ninja-ReleaseAssert/toolchain-macosx-x86_64/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/macosx (offset 12)

$ otool -l /Users/marcrasi/swift-base/build/Ninja-ReleaseAssert/toolchain-macosx-x86_64/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/macosx/libswift_Differentiation.dylib | grep ID_DYLIB -A2
          cmd LC_ID_DYLIB
      cmdsize 72
         name /usr/lib/swift/libswift_Differentiation.dylib (offset 24)

This PR fixes the problem by changing the "install name" of libswift_Differentiation.dylib to @rpath/libswift_Differentiation.dylib which makes it get found in the toolchain. Here are the same commands after rebuilding the toolchain with this PR:

$ swift run
12.0

$ otool -L /Users/marcrasi/testpackage/.build/x86_64-apple-macosx/debug/testpackage
/Users/marcrasi/testpackage/.build/x86_64-apple-macosx/debug/testpackage:
	/System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation (compatibility version 150.0.0, current version 1675.129.0)
	/usr/lib/libobjc.A.dylib (compatibility version 1.0.0, current version 228.0.0)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1281.100.1)
	@rpath/libswiftCore.dylib (compatibility version 1.0.0, current version 0.0.0)
	@rpath/libswiftDarwin.dylib (compatibility version 1.0.0, current version 0.0.0, weak)
	@rpath/libswiftSwiftOnoneSupport.dylib (compatibility version 1.0.0, current version 0.0.0)
	@rpath/libswift_Differentiation.dylib (compatibility version 1.0.0, current version 0.0.0)

$ otool -l /Users/marcrasi/swift-base/build/Ninja-ReleaseAssert/toolchain-macosx-x86_64/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/macosx/libswift_Differentiation.dylib | grep ID_DYLIB -A2
          cmd LC_ID_DYLIB
      cmdsize 64
         name @rpath/libswift_Differentiation.dylib (offset 24)

[marcrasi]

@swift-ci please test

[edymtt]

Apologies for chiming in late -- it turns out we need a different change (#34216), since we need the install name in the dylib to be absolute (while allowing this scenario to work)

[lorentey]

I think /usr/lib/swift/libFoo.dylib is the right install_name for these libraries -- after all, this is where they are expected to install when they get released, and it matches the configuration of other libraries. (Ignoring the pre-5.0 compatibility overrides in magic-symbols-for-install-name, which don't apply here.)

To test locally built libraries, our test configuration uses DYLD_LIBRARY_PATH to override the install_name. Would setting that be an acceptable workflow?