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

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.

Differential Revision: https://reviews.llvm.org/D118672

llvm/CMakeLists.txt

@@ -28,6 +28,11 @@ if (NOT PACKAGE_VERSION)
     "${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}${LLVM_VERSION_SUFFIX}")
 endif()
 
+if(NOT DEFINED LLVM_SHLIB_SYMBOL_VERSION)
+  # "Symbol version prefix for libLLVM.so"
+  set(LLVM_SHLIB_SYMBOL_VERSION "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 "

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

@@ -1 +1 @@
-LLVM_@LLVM_VERSION_MAJOR@ { global: *; };
+@LLVM_SHLIB_SYMBOL_VERSION@ { global: *; };