[CodeCompletion] Signature help for aggregate initialization.

The "parameter list" is the list of fields which should be initialized.
We introduce a new OverloadCandidate kind for this.
It starts to become harder for CC consumers to handle all the cases for
params, so I added some extra APIs on OverloadCandidate to abstract them.

Includes some basic support for designated initializers.
The same aggregate signature is shown, the current arg jumps after the
one you just initialized. This follows C99 semantics for mixed
designated/positional initializers (which clang supports in C++ as an extension)
and is also a useful prompt for C++ as C++ designated initializers must be
in order.

Related bugs:
 - https://github.com/clangd/clangd/issues/965
 - https://github.com/clangd/clangd/issues/306

Differential Revision: https://reviews.llvm.org/D116326