cmake: mark clang as needing exported symbol.

This is a nop right now, but committing this first avoids a temporary breakage
when the llvm files change to not default to exporting symbols.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178723 91177308-0d34-0410-b5e6-96231b3b80d8