When using LLVM_DISTRIBUTION_COMPONENTS, it's possible for clang's
export list to be empty. If this happens the install(EXPORTS) command
will fail, but since there isn't anything to install anyway we really
just want to skip it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@286210 91177308-0d34-0410-b5e6-96231b3b80d8