Auto merge of #125288 - nikic:update-llvm-18.1.6, r=cuviper

Update to LLVM 18.1.6

This rebases our LLVM fork on top of LLVM 18.1.6, which is planned to be the last release of the 18.x series.

Fixes #123695.
Fixes #125053.

r? `@cuviper`

Author: bors