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

llvm-svn: 201286