[clang] Move the soname declaration in a variable at the top of the file

[sylvestre.ledru]

Summary

Currently, it is a bit buried in the file even if this is
pretty important for distro.

[MaskRay]

clang/tools/libclang/CMakeLists.txt

179–180

The comment should be lifted as well.

[sylvestre.ledru]

Move the comment too

[MaskRay]

Looks great!

clang/tools/libclang/CMakeLists.txt

+# The SOVERSION should be updated only if a change is made to the libclang
+# ABI, and when it is updated, it should be updated to the current
+# LLVM_VERSION_MAJOR.
+# Please also see clang/tools/libclang/libclang.map
+set(CLANG_SONAME 13)
+
 set(SOURCES
   ARCMigrate.cpp
   BuildSystem.cpp
   CIndex.cpp
   CIndexCXX.cpp
   CIndexCodeCompletion.cpp
   CIndexDiagnostic.cpp
   CIndexHigh.cpp
if (USE_VERSION_SCRIPT)
     # The Solaris 11.4 linker supports a subset of GNU ld version scripts,
     # but requires a special option to enable it.
     if (${CMAKE_SYSTEM_NAME} MATCHES "SunOS")
       target_link_options(libclang PRIVATE "-Wl,-z,gnu-version-script-compat")
     endif()
     # Ensure that libclang.so gets rebuilt when the linker script changes.
     set_property(SOURCE ARCMigrate.cpp APPEND PROPERTY
                  OBJECT_DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/libclang.map)
 
-    # The SOVERSION should be updated only if a change is made to the libclang
-    # ABI, and when it is updated, it should be updated to the current
-    # LLVM_VERSION_MAJOR.
     set_target_properties(libclang PROPERTIES
                           VERSION ${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}${LLVM_VERSION_SUFFIX}
-                          SOVERSION 13)
+                          SOVERSION ${CLANG_SONAME})
   endif()
 endif()
 
 if(INTERNAL_INSTALL_PREFIX)
   set(LIBCLANG_HEADERS_INSTALL_DESTINATION "${INTERNAL_INSTALL_PREFIX}/include")
 else()
   set(LIBCLANG_HEADERS_INSTALL_DESTINATION include)
 endif()