[clang-repl] Fix REPL_EXTERNAL_VISIBILITY and building libclang-cpp.dll for MinGW configurations

This fixes two issues that are observed after
5111286f:

For builds with GCC with LLVM_LINK_LLVM_DYLIB=ON, we previously got
build errors, as libclang-cpp.dll suddenly only contained the
functions that were marked dllexport via REPL_EXTERNAL_VISIBILITY,
instead of all symbols as expected.

For MinGW builds with Clang, building previously succeeded (as it
used either the __attribute__((visibility("default"))) annotation or
nothing at all), and the functions were exported from libclang-cpp.dll
if that was built, but the unit test failed (as neither of those cases
made the functions exported from an EXE).

Don't use the visibility attributes on MinGW targets for these purposes;
setting default visibility only makes a difference if building with
e.g. -fvisibility=hidden, but it doesn't make the symbols exported
from an EXE.

Differential Revision: https://reviews.llvm.org/D151620