For "foo(|" [1] we requested a completion from libclang with the cursor
position just before "foo" and then filtered the function declarations
for functions matching the name "foo". This worked fine for ordinary
functions, but obviously not for constructors and functors.

Recent versions of libclang support proper function call completion with
XCursor_OverloadCandidate, so make use of that.

[1] '|' represents the cursor position

Task-number: QTCREATORBUG-14882
Task-number: QTCREATORBUG-14884
Change-Id: I9d31b3960ccff6a8b9440dbcb7ff9f5ca9f61266
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>