CMake: allow overriding CMAKE_CXX_VISIBILITY_PRESET

This allows overriding the `CMAKE_CXX_VISIBILITY_PRESET` on the command line. For example, setting the value to `default` lets PIC LLVM static libraries be converted to DSOs, without the need to rebuild LLVM with BUILD_SHARED_LIBS=ON.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D104168