Only define LLVM_EXTERNAL_VISIBILITY when building libLLVM dylib

When building LLVM static libraries, we should not make symbols more
visible than CMAKE_CXX_VISIBILITY_PRESET, since the goal may be to have
a purely hidden llvm embedded in another library. Instead, we only
define LLVM_EXTERNAL_VISIBILITY for the dynamic library build (when
LLVM_BUILD_LLVM_DYLIB=YES).

Differential Revision: https://reviews.llvm.org/D113610