rustbuild: Update LLVM and enable ThinLTO
This commit updates LLVM to fix #45511 (https://reviews.llvm.org/D39981) and
also reenables ThinLTO for libtest now that we shouldn't hit #45768. This also
opportunistically enables ThinLTO for libstd which was previously blocked
(#45661) on test failures related to debuginfo with a presumed cause of #45511.
Closes #45511