debuginfo: LTO triggers assertion in `llvm::LexicalScopes::getOrCreateRegularScope()`

[michaelwoerister]

Steps to reproduce:

1. Build rustc with RUSTFLAGS_STAGE2=-g make rustc-stage2
2. Try to compile a simple hello-world program with rustc ./ltocrash.rs -g -C lto

fn main() {
    println!("Hello World!");
}

1. Observe that rustc crashes with the following error:

rustc: /home/mw/rust/src/llvm/lib/CodeGen/LexicalScopes.cpp:179: llvm::LexicalScope* llvm::LexicalScopes::getOrCreateRegularScope(llvm::MDNode*): Assertion `DISubprogram(Scope).describes(MF->getFunction())' failed.

A very similar issue (#17201) occurred in non-LTO cases.

[alexcrichton]

This no longer happens on stable, so closing as fixed, yay!