The LLVM_ABI_REVISION variable is intended to be used for release
candidates which introduce an ABI change to a shared library. This
variable can be specified per library, so there is not one global value
for all of LLVM.
For example, if we LLVM X.0.0-rc2 introduces an ABI change for a library
compared with LLVM X.0.0-rc1, then the LLVM_ABI_REVISION number for
library will be incremented by 1.
In the main branch, LLVM_ABI_REVISION should always be 0, it is only
meant to be used in the release branch.