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>

llvm-svn: 208263
diff --git a/llvm/test/Assembler/private-protected-alias.ll b/llvm/test/Assembler/private-protected-alias.ll
new file mode 100644
index 0000000..a72c248
--- /dev/null
+++ b/llvm/test/Assembler/private-protected-alias.ll
@@ -0,0 +1,6 @@
+; RUN: not llvm-as < %s -o /dev/null 2>&1 | FileCheck %s
+
+@global = global i32 0
+
+@alias = protected alias private i32* @global
+; CHECK: symbol with local linkage must have default visibility