[llvm] ThinLTOBitcodeWriter: Emit __cfi_check to full LTO part of bitcode file. (PR #154833)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

My initial testing showed this was working... but further testing shows it's not completely reliable.  Example:

```
$ cat a.c
int a = 3;
$ cat b.c
void g(){}
$ clang a.c b.c -flto=thin -fsanitize=cfi -fsanitize-cfi-cross-dso -O2 -c -fvisibility=default
$ clang a.o b.o -flto=thin -fsanitize=cfi -fsanitize-cfi-cross-dso -O2 -fuse-ld=lld -shared -Wl,-save-temps -fvisibility=default
$ opt -S a.out.0.0.preopt.bc
```

https://github.com/llvm/llvm-project/pull/154833