swift234

[ClangImporter] Avoid Clang's isModuleVisible.

Swift sometimes has a different notion of visibility, even for Clang modules.
In particular, this was causing the C version of NSUTF8StringEncoding
(from Foundation) to show up in AppKit, bypassing the Swift version in the
overlay.

We still want to search for redeclarations of functions, typedefs, and globals,
since these are often repeated in multiple modules without harm. The decl will
still have a "home" module, but this should allow them to be found by lookup.

<rdar://problem/16396994> and possibly <rdar://problem/14665250>

Swift SVN r16762