Backport fix for clangd#1951 to the 18.x branch

[HighCommander4]

I would like to propose backporting the fix for clangd/clangd#1951 to the 18.x branch.

The fix is for a regression that affects clangd 17. The symptom is a crash when processing system headers with some standard library versions, so I'd like to avoid having it affect clangd 18 as well.

[HighCommander4]

/cherry-pick e6e53ca

[llvmbot]

@llvm/issue-subscribers-clangd

Author: Nathan Ridge (HighCommander4)

I would like to propose backporting the fix for https://github.com/clangd/clangd/issues/1951 to the 18.x branch.

The fix is for a regression that affects clangd 17. The symptom is a crash when processing system headers with some standard library versions, so I'd like to avoid having it affect clangd 18 as well.

[llvmbot]

/pull-request #84117