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>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208261 91177308-0d34-0410-b5e6-96231b3b80d8
LTO: Assert visibility of local linkage when merging symbols

`ModuleLinker::getLinkageResult()` shouldn't create symbols with local
linkage and non-default visibility -- in fact, symbols with local
linkage shouldn't be merged at all.  Assert to that effect.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208262 91177308-0d34-0410-b5e6-96231b3b80d8
IR: Don't allow non-default visibility on local linkage

Visibilities of `hidden` and `protected` are meaningless for symbols
with local linkage.

  - Change the assembler to reject non-default visibility on symbols
    with local linkage.

  - Change the bitcode reader to auto-upgrade `hidden` and `protected`
    to `default` when the linkage is local.

  - Update LangRef.

<rdar://problem/16141113>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208263 91177308-0d34-0410-b5e6-96231b3b80d8
GlobalValue: Assert symbols with local linkage have default visibility

The change to ExtractGV.cpp has no functionality change except to avoid
the asserts.  Existing testcases already cover this, so I didn't add a
new one.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208264 91177308-0d34-0410-b5e6-96231b3b80d8
Back out r208257 while I investigate tester failures.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208267 91177308-0d34-0410-b5e6-96231b3b80d8