[CodeGen] Keep track of eagerly emitted globals

An inline virtual function must be emitted, but we need to remember
it and emit the same definition again in the future in case later
LLVM optimizations stripped it from the Module. The added test case
shows the problem; before this patch, it would fail with:
    Symbols not found: [ _ZN1AD0Ev, _ZN1AD1Ev ]

This reapplies commit f8dadefd, reverted in commit 0e17372b, but
disables RTTI in the test to avoid problems on Windows.

Differential Revision: https://reviews.llvm.org/D156537