[clang] [clang-tools-extra] [clangd] Autocomplete fixes for methods (PR #165916)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> At the moment, `IsAddressOfOperand` is needed as `FunctionCanBeCalled` is not able to determine whether a completion is a call or a pointer.

I wrote up what I had in mind in https://github.com/HighCommander4/llvm-project/commit/27df471cb6e0ad83b3d9a9a4a38998686ffb7c70.

If it looks reasonable to you, please feel free to cherry-pick it onto the PR branch.

> let me know if you want me to remove those modification and open a different PR for [clangd/clangd#880](https://github.com/clangd/clangd/issues/880).

It's fine to leave that in this PR, since it's also related to proper handling of function completions in a declaration context.

(I'm continuing to look through the patch and will have a few more comments but I expect they will be minor.)

https://github.com/llvm/llvm-project/pull/165916