Introduce llvm-enable-rtti option

[catap]

This option is controls when LLVM build adds -fno-rtti. Some details available here: https://www.llvm.org/docs/HowToSetUpLLVMStyleRTTI.html

When this option is enabled, llvm build produces a bit more symbols:

rust % nm build/x86_64-apple-darwin/llvm/lib/libLLVMCore.a | grep llvm10CallbackVHE
0000000000007120 S __ZTIN4llvm10CallbackVHE
0000000000007240 S __ZTSN4llvm10CallbackVHE
00000000000070e8 S __ZTVN4llvm10CallbackVHE

and just to compare the same build without this option (current behaviour):

rust % nm build/x86_64-apple-darwin/llvm/lib/libLLVMCore.a | grep llvm10CallbackVHE
00000000000070b8 S __ZTVN4llvm10CallbackVHE

An attempt to bootstrap rust with clang which was built with enabled RTTI may fail with linking error like:

 = note: dyld: Symbol not found: __ZTIN4llvm10CallbackVHE
            Referenced from: /opt/local/libexec/llvm-9.0/bin/../lib/libclang-cpp.dylib
            Expected in: .../build/x86_64-apple-darwin/stage2/lib/libLLVM.dylib
           in /opt/local/libexec/llvm-9.0/bin/../lib/libclang-cpp.dylib

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Mark-Simulacrum (or someone else) soon.

Please see the contribution instructions for more information.

[bjorn3]

What is the exact purpose of this option? Why would someone want to enable it?

[catap]

@bjorn3 I've tried to build rustc by clang which was compiled without -fno-rtti and it fails because rust's LLVM is built with -fno-rtti. So, it works fine but at stage2 build fails because it can't link anything with provided errors.

As soon as I added this option and enabled it to this build => it linked and works.

[Mark-Simulacrum]

I'm going to close this in favor of #93756. Thanks!