Auto merge of #111364 - cuviper:unhack-thinlto, r=nikic

Remove the ThinLTO CU hack

This reverts #46722, commit e0ab5d5.

Since #111167, commit 10b69dd, we are
generating DWARF subprograms in a way that is meant to be more compatible
with LLVM's expectations, so hopefully we don't need this workaround
rewriting CUs anymore.

Author: bors