[cmake] Append LLVM_VERSION_SUFFIX to SOVERSION

Append LLVM_VERSION_SUFFIX to SOVERSION. This makes it possible
to use the suffix to differentiate binary-incompatible versions
of LLVM built via BUILD_SHARED_LIBS.

We are planning to use this to temporarily preserve ABI-incompatible
variants of LLVM while switching the system between them, e.g. when
rebuilding the system to use libc++. Normally this would mean that once
LLVM is rebuilt using libc++ all the reverse dependencies become
immediately broken. Using a distinct SOVERSION allows us to preserve
the ABI compatibility before all the packages are rebuilt.

Differential Revision: https://reviews.llvm.org/D39939

llvm-svn: 339286