[Build] Turn off -undefined-version when building with lld

Newer lld defaults to `-no-undefined-version`, which breaks the build
because of various symbols that are (a) versioned and (b) not always
defined (depending on platform).

rdar://123436280

Author: al45tair

llvm/cmake/modules/AddLLVM.cmake

@@ -254,6 +254,12 @@ if (NOT DEFINED LLVM_LINKER_DETECTED AND NOT WIN32)
 endif()
 
 function(add_link_opts target_name)
+  # Newer lld defaults to no-undefined-version, which we don't expect
+  if(LLVM_LINKER_IS_LLD)
+    set_property(TARGET ${target_name} APPEND_STRING PROPERTY
+      LINK_FLAGS " -Wl,-undefined-version")
+  endif()
+
   get_llvm_distribution(${target_name} in_distribution in_distribution_var)
   if(NOT in_distribution)
     # Don't LTO optimize targets that aren't part of any distribution.