libclang: Make shared object symbol exporting by default

https://reviews.llvm.org/D74564 enabled static building for libclang,
and for non CMake consumers they had to set the `CMAKE_EXPORTS` define
when consuming libclang.

This commit makes the non CMake users of the static building have to define `CMAKE_NO_EXPORTS`.

Differential Revision: https://reviews.llvm.org/D74907