[clangd] Handle variable templates consistently with class templates in code completion (#85740)

The option --function-arg-placeholders=0 results in placeholders being
omitted for class template argument lists. This patch extends the same
treatment to variable template argument lists.

Fixes https://github.com/clangd/clangd/issues/1976

GitOrigin-RevId: 807fd07646646d7f96bd3561e79435b5d7bb4a64
2 files changed