LTO: Check local linkage first

Since visibility is meaningless for symbols with local linkage, check
local linkage before visibility when setting symbol attributes.

When linkage is `internal` and the visibility is `hidden`, the exposed
attribute is now `LTO_SYMBOL_SCOPE_INTERNAL` instead of
`LTO_SYMBOL_SCOPE_HIDDEN`.  Although the bitfield allows *both* to be
specified, the combination is nonsense anyway.

Given changes (in progress) to drop visibility when a symbol has local
linkage, this almost has no functionality change: it's mostly a cleanup
to clarify the logic.

The exception is when something has `appending` linkage.  Before this
change, such symbols would be advertised as `LTO_SYMBOL_SCOPE_INTERNAL`;
now, they'll be given `LTO_SYMBOL_SCOPE_COMMON`.

Unfortunately this is really awkward to test.  This only changes what we
advertise to linkers (before running LTO), not what the final object
looks like.  In theory I could add `DEBUG` output to `llvm-lto` (and
test with "REQUIRES: asserts"), but follow-up commits to disallow
`internal hidden` simplify this anyway.

<rdar://problem/16141113>

llvm-svn: 208261

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