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/Transforms/GlobalOpt/alias-used-section.ll b/llvm/test/Transforms/GlobalOpt/alias-used-section.ll
index 987c4a4..1217937 100644
--- a/llvm/test/Transforms/GlobalOpt/alias-used-section.ll
+++ b/llvm/test/Transforms/GlobalOpt/alias-used-section.ll
@@ -1,8 +1,8 @@
 ; RUN: opt -S -globalopt < %s | FileCheck %s
 
 @_Z17in_custom_section = internal global i8 42, section "CUSTOM"
-@in_custom_section = protected dllexport alias internal i8* @_Z17in_custom_section
+@in_custom_section = dllexport alias internal i8* @_Z17in_custom_section
 
-; CHECK: @in_custom_section = internal protected dllexport global i8 42, section "CUSTOM"
+; CHECK: @in_custom_section = internal dllexport global i8 42, section "CUSTOM"
 
 @llvm.used = appending global [1 x i8*] [i8* @in_custom_section], section "llvm.metadata"