[llvm] Be explicit about what libstdc++ C++11 ABI to use (PR #154447)

[Shilei Tian via llvm-commits]

shiltian wrote:

Adding one data point: I'm using Clang 20.1.8 on Ubuntu 24.04. `-gen-register-bank` and `-gen-register-info` are extremely slow here, taking about 1.5 hours.

https://github.com/llvm/llvm-project/pull/154447