[CodeGen] Fix assertion on referencing constexpr Obj-C object with ARC.

[CodeGen] Fix assertion on referencing constexpr Obj-C object with ARC.

Failed assertion is
> Assertion failed: ((ND->isUsed(false) || !isa<VarDecl>(ND) || !E->getLocation().isValid()) && "Should not use decl without marking it used!"), function EmitDeclRefLValue, file llvm-project/clang/lib/CodeGen/CGExpr.cpp, line 2437.

`EmitDeclRefLValue` mentions
> // A DeclRefExpr for a reference initialized by a constant expression can
> // appear without being odr-used. Directly emit the constant initializer.

The fix is to use the similar approach for non-references as for references. It
is achieved by trying to emit a constant before we attempt to load non-odr-used
variable as LValue.

rdar://problem/40650504

Reviewers: ahatanak, rjmccall

Reviewed By: rjmccall

Subscribers: dexonsmith, erik.pilkington, cfe-commits

Differential Revision: https://reviews.llvm.org/D53674

llvm-svn: 345903