[cmake][NFC] Configuration for libLLVM.so symbol versioning

[stephenneuendorffer]

Summary

Symbol versioning can prevent unintented install-time conflicts
between different llvm versions. Users may need to override this
for particular products (e.g. Julia), but this requires carrying
a source code patch. This patch moves this ability to a
configuration option. NFC for existing usage.

[vchuravy]

This is great, we have been carrying https://github.com/JuliaLang/julia/blob/a0093d2ffb7ba1d35071543e581c26b96a772d39/deps/patches/llvm7-symver-jlprefix.patch

llvm/CMakeLists.txt

if(NOT DEFINED LLVM_VERSION_SUFFIX)
   set(LLVM_VERSION_SUFFIX git)
 endif()
 
 if (NOT PACKAGE_VERSION)
   set(PACKAGE_VERSION
     "${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}${LLVM_VERSION_SUFFIX}")
 endif()
 
+if(NOT DEFINED LLVM_SHLIB_SYMBOL_PREFIX)
+  # "Symbol version prefix for libLLVM.so"
+  set(LLVM_SHLIB_SYMBOL_PREFIX "LLVM_${LLVM_VERSION_MAJOR}")
+endif()
+
 if ((CMAKE_GENERATOR MATCHES "Visual Studio") AND (MSVC_TOOLSET_VERSION LESS 142) AND (CMAKE_GENERATOR_TOOLSET STREQUAL ""))
   message(WARNING "Visual Studio generators use the x86 host compiler by "
                   "default, even for 64-bit targets. This can result in linker "
                   "instability and out of memory errors. To use the 64-bit "
                   "host compiler, pass -Thost=x64 on the CMake command line.")
 endif()
 
 if (CMAKE_GENERATOR STREQUAL "Xcode" AND NOT CMAKE_OSX_ARCHITECTURES)

llvm/tools/llvm-shlib/simple_version_script.map.in

-LLVM_@LLVM_VERSION_MAJOR@ { global: *; };
+@LLVM_SHLIB_SYMBOL_PREFIX@ { global: *; };