[llvm-shlib] Change libLLVM-$MAJOR.so symlink to point to versioned SO (#82660)

This symlink was added in 91a3846 to
maintain backwards compatibility, but it needs to point to
libLLVM.so.$MAJOR.$MINOR rather than libLLVM.so. This works better for
distros that ship libLLVM.so and libLLVM.so.$MAJOR.$MINOR in separate
packages and also prevents mistakes like
libLLVM-19.so -> libLLVM.so -> libLLVM.so.18.1

Fixes #82647

Author: tstellar

llvm/cmake/modules/AddLLVM.cmake

@@ -2074,7 +2074,7 @@ function(add_lit_testsuites project directory)
 endfunction()
 
 function(llvm_install_library_symlink name dest type)
-  cmake_parse_arguments(ARG "" "COMPONENT" "" ${ARGN})
+  cmake_parse_arguments(ARG "" "COMPONENT;SOVERSION" "" ${ARGN})
   foreach(path ${CMAKE_MODULE_PATH})
     if(EXISTS ${path}/LLVMInstallSymlink.cmake)
       set(INSTALL_SYMLINK ${path}/LLVMInstallSymlink.cmake)
@@ -2088,7 +2088,11 @@ function(llvm_install_library_symlink name dest type)
   endif()
 
   set(full_name ${CMAKE_${type}_LIBRARY_PREFIX}${name}${CMAKE_${type}_LIBRARY_SUFFIX})
-  set(full_dest ${CMAKE_${type}_LIBRARY_PREFIX}${dest}${CMAKE_${type}_LIBRARY_SUFFIX})
+  if (ARG_SOVERSION)
+    set(full_dest ${CMAKE_${type}_LIBRARY_PREFIX}${dest}${CMAKE_${type}_LIBRARY_SUFFIX}.${ARG_SOVERSION})
+  else()
+    set(full_dest ${CMAKE_${type}_LIBRARY_PREFIX}${dest}${CMAKE_${type}_LIBRARY_SUFFIX})
+  endif()
 
   if(LLVM_USE_SYMLINKS)
     set(LLVM_LINK_OR_COPY create_symlink)

llvm/tools/llvm-shlib/CMakeLists.txt

@@ -35,8 +35,8 @@ if(LLVM_BUILD_LLVM_DYLIB)
   endif()
   add_llvm_library(LLVM SHARED DISABLE_LLVM_LINK_LLVM_DYLIB OUTPUT_NAME LLVM ${INSTALL_WITH_TOOLCHAIN} ${SOURCES})
   # Add symlink for backwards compatibility with old library name
-  get_target_property(LLVM_DYLIB_FILENAME LLVM OUTPUT_NAME)
-  llvm_install_library_symlink(LLVM-${LLVM_VERSION_MAJOR}${LLVM_VERSION_SUFFIX} ${LLVM_DYLIB_FILENAME} SHARED COMPONENT LLVM)
+  get_target_property(LLVM_DYLIB_SOVERSION LLVM SOVERSION)
+  llvm_install_library_symlink(LLVM-${LLVM_VERSION_MAJOR}${LLVM_VERSION_SUFFIX} LLVM SHARED COMPONENT LLVM SOVERSION ${LLVM_DYLIB_SOVERSION})
 
   list(REMOVE_DUPLICATES LIB_NAMES)
   if("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")