ThinLTO dylib issue 105637 reproduction

Interesting things on nightly-2022-12-12 (which still has rustc_driver built with ThinLTO on linux):

1. the wrong panic hook is to be called
2. symbols are duplicated with -Zdylib-lto, some of libstd's static, like the mutable std::panicking::HOOK storing the current panic hook.

Duplicates the structure between rustc's main binary wrapper and rustc_driver's dylib containing the actual code, to reproduce issue #105637.

Example without ThinLTO on dylib

cargo clean && RUSTFLAGS="-Cprefer-dynamic" cargo run -q

Output:

rustc_main - updating panic hook
rustc_driver - DEFAULT_HOOK forced, installing new panic hook
Compiling, about to panic.
rustc_driver - panic hook executing

rustc_driver's panic hook is executed.

Looking at the dylib panic hook symbols, it's undefined:

nm ./target/debug/librustc_driver.so | rustfilt | grep "std::panicking::HOOK"
                 U std::panicking::HOOK

Example with ThinLTO

cargo clean && RUSTFLAGS="-Cprefer-dynamic -Zdylib-lto -Cembed-bitcode -Clto=thin" cargo run -q

Output:

rustc_main - updating panic hook
rustc_driver - DEFAULT_HOOK forced, installing new panic hook
Compiling, about to panic.
rustc_main - panic hook executing

rustc_main's panic hook is executed.

Looking at the dylib panic hook symbols, it's a local BSS symbol:

nm ./target/debug/librustc_driver.so | rustfilt | grep "std::panicking::HOOK"
00000000000e4118 b std::panicking::HOOK