Add support for inline parameter hints

[wtao137]

Some IDEs have a feature called "inline parameter hints", where the client renders annotations showing parameter names inline in function calls.

Screenshot to illustrate:

This is now corresponding feature called Inlay Hints in the lnaguage server protocal https://github.com/microsoft/language-server-protocol/issues/956 .

And clangd has implemented this for c/c++. As a result, I'm wondering if it's possible to implement it in ccls, since having it will be quite useful.

The original issue for clangd: https://github.com/clangd/clangd/issues/313