Revert r345562: "PR23833, DR2140: an lvalue-to-rvalue conversion on a glvalue of type"

This exposes a (known) CodeGen bug: it can't cope with emitting lvalue
expressions that denote non-odr-used but usable-in-constant-expression
variables. See PR39528 for a testcase.

Reverted for now until that issue can be fixed.

llvm-svn: 346065