[clangd][CodeComplete] Improve FunctionCanBeCall

From two aspects:

- For function templates, emit additional template argument
placeholders in the context where it can't be a call in order
to specify an instantiation explicitly.

- Consider expressions with base type specifier such as
'Derived().Base::foo^' a function call.

Reviewed By: nridge

Differential Revision: https://reviews.llvm.org/D156605