Auto merge of #42971 - stepancheg:ir-demangle, r=nagisa

Auto merge of #42971 - stepancheg:ir-demangle, r=nagisa

When writing LLVM IR output demangled fn name in comments

`--emit=llvm-ir` looks like this now:

```
; <alloc::vec::Vec<T> as core::ops::index::IndexMut<core::ops::range::RangeFull>>::index_mut
; Function Attrs: inlinehint uwtable
define internal { i8*, i64 } @"_ZN106_$LT$alloc..vec..Vec$LT$T$GT$$u20$as$u20$core..ops..index..IndexMut$LT$core..ops..range..RangeFull$GT$$GT$9index_mut17h7f7b576609f30262E"(%"alloc::vec::Vec<u8>"* dereferenceable(24)) unnamed_addr #0 {
start:
  ...
```

cc https://github.com/integer32llc/rust-playground/issues/15