[clang][clangd] Improve signature help for variadic functions.

This covers both C-style variadic functions and template variadic w/
parameter packs.

Previously we would return no signatures when working with template
variadic functions once activeParameter reached the position of the
parameter pack (except when it was the only param, then we'd still
show it when no arguments were given). With this commit, we now show
signathure help correctly.

Additionally, this commit fixes the activeParameter value in LSP output
of clangd in the presence of variadic functions (both kinds). LSP does
not allow the activeParamter to be higher than the number of parameters
in the active signature. With "..." or parameter pack being just one
argument, for all but first argument passed to "..." we'd report
incorrect activeParameter value. Clients such as VSCode would then treat
it as 0, as suggested in the spec) and highlight the wrong parameter.

In the future, we should add support for per-signature activeParamter
value, which exists in LSP since 3.16.0. This is not part of this
commit.

Differential Revision: https://reviews.llvm.org/D111318

0 files changed, 0 insertions, 0 deletions