Speculatively implement a tweak to the C++0x overload resolution rules

Speculatively implement a tweak to the C++0x overload resolution rules
for reference binding (C++ [over.rank.ics]p3b1sb4), so that we prefer
the binding of an lvalue reference to a function lvalue over the
binding of an rvalue reference. This change resolves the ambiguity
with std::forward and lvalue references to function types in a way
that seems consistent with the original rvalue references proposal.

My proposed wording for this change is shown in
isBetterReferenceBindingKind(); we'll try to get this change adopted
in the C++0x working paper as well.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@124236 91177308-0d34-0410-b5e6-96231b3b80d8