Use the proper install name for back-deployed concurrency on macCatalyst.

Fixes rdar://84393581.

Author: DougGregor

stdlib/public/Concurrency/linker-support/magic-symbols-for-install-name.c

@@ -124,6 +124,12 @@
   RPATH_INSTALL_NAME_DIRECTIVE(11,  4)
   RPATH_INSTALL_NAME_DIRECTIVE(11,  5)
   RPATH_INSTALL_NAME_DIRECTIVE(11,  6)
+
+ // Link against @rpath/libswift_Concurrency.dylib for macCatalyst < 15.0.
+SWIFT_RUNTIME_EXPORT const char ld_previous_macCatalyst
+__asm("$ld$previous$@rpath/libswift_Concurrency.dylib$$6$1.0$15.0$");
+
+const char ld_previous_macCatalyst = 0;
 #else
   #error Unknown target.
 #endif

test/Concurrency/Backdeploy/linking.swift

@@ -7,6 +7,12 @@
 // RUN: otool -L %t/linking_rpath | %FileCheck -check-prefix CHECK-RPATH %s
 // RUN: otool -L %t/linking_rpath_old | %FileCheck -check-prefix CHECK-RPATH %s
 
+// RUN: %target-build-swift -target x86_64-apple-ios15.0-macabi %s -o %t/linking_direct
+// RUN: %target-build-swift -target x86_64-apple-ios14.0-macabi %s -o %t/linking_rpath
+
+// RUN: otool -L %t/linking_direct | %FileCheck -check-prefix CHECK-DIRECT %s
+// RUN: otool -L %t/linking_rpath | %FileCheck -check-prefix CHECK-RPATH %s
+
 // REQUIRES: OS=macosx
 // REQUIRES: CPU=x86_64