cmake: Store LLVM_VERSION_SUFFIX in LLVMConfig.cmake

Store LLVM_VERSION_SUFFIX along with other version components
in LLVMConfig.cmake. This fixes preserving the suffix set while building
LLVM to stand-alone builds of other components, e.g. clang,
and therefore improves uniformity between the two build models.

Given that there is no apparent reason to omit this part of version,
that it is distributed to subprojects when building as part of LLVM
and that it is included in LLVM_PACKAGE_VERSION, I think it was omitted
accidentally rather than done on purpose.

Differential Revision: https://reviews.llvm.org/D43701

llvm-svn: 339285