Replace leading typed text when completing function implementation

Summary:
When executing function-implementation auto-completion, this will now
replace and leading typed text that matches the proposed completion
instead of duplicating it.

BUG: 384710

Test Plan:

int Foo(int thing);
struct Bar
{
  void Method(int thing);
}

Start typing int Foo or Bar::Meth and then execute the offered completion.

Reviewers: KDevelop, mwolff

Reviewed By: KDevelop, mwolff

Subscribers: kfunk, brauch, mwolff, apol, kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D16326