Rollup merge of #113688 - krasimirgg:llvm-17-small-string, r=nikic

llvm-wrapper: update for LLVM API change

No functional changes intended.

Adds an include for `llvm::SmallString`. Previously, this must have been implicitly provided by some of the existing headers. With recent LLVM changes, not anymore:
https://buildkite.com/llvm-project/rust-llvm-integrate-prototype/builds/20776#01895448-44a4-4a1e-8407-9d41d0186132/209-690

Author: matthiaskrgr