[clangd] Add code completion of param name on /* inside function calls.

For example, if you have:
  void foo(int bar);
  foo(/*^
it should auto-complete to "bar=".

Because Sema callbacks for code completion in comments happen before we
have an AST we need to cheat in clangd by detecting completion on /*
before, moving cursor back by two characters, then running a simplified
verion of SignatureHelp to extract argument name(s) from possible
overloads.

Differential Revision: https://reviews.llvm.org/D110823