[CMake][MS] libclang: Invalidate LLVM_EXPORTED_SYMBOL_FILE to suppress LNK4197.

Each functions is exported as "dllexport" in include/clang-c.
See also KB835326.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199799 91177308-0d34-0410-b5e6-96231b3b80d8