[cmake] Set in LLVM_EXPORTED_SYMBOL_FILE PrintFunctionNames/CMakeLists.txt.

The corresponding Makefile sets it too. Also tweak add_clang_library to support
LLVM_EXPORTED_SYMBOL_FILE for modules.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@198185 91177308-0d34-0410-b5e6-96231b3b80d8
Fix typo in comment.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@198186 91177308-0d34-0410-b5e6-96231b3b80d8