This patch ensures we can find decls in submodules during expression
evaluation. Previously, submodules would have all their decls marked as
`Hidden`. When Clang asked LLDB for decls, it would see them in the
submodule but `clang::Sema` would reject them because they weren't
`Visible` (specifically, `getAcceptableDecl` would fail during
`CppNameLookup`). Here we just mark the submodule as visible to work
around this problem.
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