[llvm] RuntimeLibcalls: Make getLibcallImplName static. NFC. (PR #147919)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Jul 10, 12:17 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/147919).


https://github.com/llvm/llvm-project/pull/147919