Skip passing /natvis to lld-link until supported.

[MaulingMonkey]

Overview

Teaching rustc about MSVC's undocumented linker flag, /NATVIS, broke rustc's compatability with LLVM's lld-link frontend, as it does not recognize the flag. This pull request works around the problem by excluding lld-link by name. @retep998 discovered this regression.

Possible Issues

• Other linkers that try to be compatible with the MSVC linker flavor may also be broken and in need of workarounds.
• Warning about the workaround may be overkill for a minor reduction in debug functionality.
• Depending on how long this workaround sticks around, it may eventually be preferred to version check lld-link instead of assuming all versions are incompatible.

Relevant issues

• Broke in Embed MSVC .natvis files into .pdbs and mangle debuginfo for &str, *T, and [T]. #43221 Embed MSVC .natvis files into .pdbs and mangle debuginfo for &str, *T, and [T].
• LLVM patched in llvm-mirror/lld@27b9c42 to ignore the flag instead of erroring.

r? @michaelwoerister

[rust-highfive]

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

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[michaelwoerister]

@bors r+

Thanks a lot!

[bors]

📌 Commit 995f0bd has been approved by michaelwoerister

[michaelwoerister]

@bors rollup

[Boddlnagg]

This seems to be supported in lld now: https://reviews.llvm.org/rL327895

[MaulingMonkey]

Nice! I can create a new PR simply reverting this change if we're OK with dropping support for LLVM 5 lld-link? LLVM 6 was released 8 Mar 2018, and should at worst ignore /natvis...

Alternatively I can try to invoke lld-link.exe --version and parse that, but that seems like a lot of brittle hard to test code for minimal gain...

EDIT: Someone beat me to it: #59441 - rustc 1.35.0+ should start using the flag again and LLVM 7+ should actually embed the PDBs instead of just ignoring them like LLVM 6+.