[cmake] Update SOVERSION for the new versioning scheme

[mgorny]

Summary

Update SOVERSION to use just the major version number rather than
major+minor, to match the new versioning scheme where only major is used
to indicate API/ABI version.

Since two-digit SOVERSIONs were introduced post 3.9 branching, this
change does not risk any SOVERSION collisions. In the past,
two-component X.Y SOVERSIONs were shortly used but those will not
interfere with the new ones since the new versions start at 4.

[beanz]

LGTM.

[mgorny]

@hans, could this be backported to 4.0 branch, please?

llvm/trunk/cmake/modules/AddLLVM.cmake

if(WIN32)
         )
     endif()
 
     # Set SOVERSION on shared libraries that lack explicit SONAME
     # specifier, on *nix systems that are not Darwin.
     if(UNIX AND NOT APPLE AND NOT ARG_SONAME)
       set_target_properties(${name}
         PROPERTIES
-    # Concatenate the version numbers since ldconfig expects exactly
-    # one component indicating the ABI version, while LLVM uses
-    # major+minor for that.
-        SOVERSION ${LLVM_VERSION_MAJOR}${LLVM_VERSION_MINOR}
-        VERSION ${LLVM_VERSION_MAJOR}${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}${LLVM_VERSION_SUFFIX})
+        # Since 4.0.0, the ABI version is indicated by the major version
+        SOVERSION ${LLVM_VERSION_MAJOR}
+        VERSION ${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}${LLVM_VERSION_SUFFIX})
     endif()
   endif()
 
   if(ARG_MODULE OR ARG_SHARED)
     # Do not add -Dname_EXPORTS to the command-line when building files in this
     # target. Doing so is actively harmful for the modules build because it
     # creates extra module variants, and not useful because we don't use these
     # macros.