[patch] First step to fix pr11866 during LTO

[Rafael Espíndola]

It turns out to be fairly hard to fix pr11866 without LTO since we
cannot mark a function hidden until we are sure there are no other
uses or definitions of it in other translation units because of the
way visibilities are merged.

During LTO on the other hand, the linker can easily let us know  that
all it wants a symbol for is to put it in a symbol table. This is
exactly what gold provides with the LDPR_PREVAILING_DEF_IRONLY_EXP
resolution.

What the attached patch does is forward that to LTO and then to
internalize. Internalize then uses the information to hide the symbol
when possible. In this patch I have only implemented hiding when the
symbol is also unnamed_addr. In a followup I will refactor some code
out of GlobalOpt so that we can also hide symbols that are not
unnamed_addr but whose address doesn't leak.

Cheers,
Rafael
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