GlobalValue: Testcase for hidden visibility and local linkage

This is a testcase for r209227, a change in LLVM that automatically sets
visibility to default when the linkage is changed to local (rather than
asserting).

What this testcase triggers is hard to reproduce otherwise:  the
`GlobalValue` is created (with non-local linkage), the visibility is set
to hidden, and then the linkage is set to local.

PR19760