[patch] Fix PR16060

[Rafael Espíndola]

The testcase in PR16060 points out that while template arguments can
show that a type is not externally visible, the standards still says
they have external linkage.

In terms of our implementation, it means that we should merge just the
isExternallyVisible bit, not the formal linkage.

This patch does just that. Suggestion for a less confusing name than
LinkageVisibility are welcome!

Cheers,
Rafael
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