Inline foot gun into only valid use.

Symbol had both Visibility and getVisibility() and they had different
meanings. That is just too easy to get wrong.

getVisibility() would compute the visibility of a particular symbol
(foo in bar.o), and Visibility stores the computed value we will put
in the output.

There is only one case when we want what getVisibility() provides, so
inline it.

llvm-svn: 322590