[LLDB] Allow specifying a custom exports file (#68013)

LLDB has the cmake flag `LLDB_EXPORT_ALL_SYMBOLS` that exports the lldb,
lldb_private namespaces, as well as other symbols like python and lua
(see `lldb/source/API/liblldb-private.exports`). However, not all
symbols in lldb fall into these categories and in order to get access to
some symbols that live in plugin folders (like dwarf parsing symbols),
it's useful to be able to specify a custom exports file giving more
control to the developer using lldb as a library.

This adds the new cmake flag `LLDB_EXPORT_ALL_SYMBOLS_EXPORTS_FILE` that
is used when `LLDB_EXPORT_ALL_SYMBOLS` is enabled to specify that custom
exports file.

This is a follow up of https://github.com/llvm/llvm-project/pull/67851