GlobalOpt: Aliases don't have sections, don't copy them when replacing

As defined in LangRef, aliases do not have sections.  However, LLVM's
GlobalAlias class inherits from GlobalValue, which means we can read and
set its section.  We should probably ban that as a separate change,
since it doesn't make much sense for an alias to have a section that
differs from its aliasee.

Fixes PR18757, where the section was being lost on the global in code
from Clang like:

extern "C" {
__attribute__((used, section("CUSTOM"))) static int in_custom_section;
}

Reviewers: rafael.espindola

Differential Revision: http://llvm-reviews.chandlerc.com/D2758

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201286 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/IPO/GlobalOpt.cpp

@@ -3018,7 +3018,8 @@ bool GlobalOpt::OptimizeGlobalAliases(Module &M) {
       // Give the aliasee the name, linkage and other attributes of the alias.
       Target->takeName(J);
       Target->setLinkage(J->getLinkage());
-      Target->GlobalValue::copyAttributesFrom(J);
+      Target->setVisibility(J->getVisibility());
+      Target->setDLLStorageClass(J->getDLLStorageClass());
 
       if (Used.usedErase(J))
         Used.usedInsert(Target);

test/Transforms/GlobalOpt/alias-resolve.ll

@@ -1,7 +1,4 @@
-; We use a temporary file so that the test fails when opt crashes.
-
-; RUN: opt < %s -globalopt -S > %t
-; RUN: FileCheck %s < %t
+; RUN: opt < %s -globalopt -S | FileCheck %s
 
 @foo1 = alias void ()* @foo2
 ; CHECK: @foo1 = alias void ()* @foo2

test/Transforms/GlobalOpt/alias-used-section.ll

@@ -0,0 +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
+
+; CHECK: @in_custom_section = internal protected dllexport global i8 42, section "CUSTOM"
+
+@llvm.used = appending global [1 x i8*] [i8* @in_custom_section], section "llvm.metadata"