[patch lto/c++/middle-end/windows]: Fix handling of dllexport for LTO as externally_visible

[Jan Hubicka]

You need to only update cgraph_externally_visible_p and
varpool_externally_visible_p in ipa.c and
process_function_and_variable_attributes (that one is missing in your patch and
I think it is needed to solve same scenarios as
process_function_and_variable_attributes was introduced for).

You would need to check the flag on all those places, too and attribute lookup
should be cheap given that those functions are called constant amount of times
per symbol.

Honza