Clang CodeCompletion: No auto-completion for numbers

Summary: When typing a number, code-completion is undesirable in my opinion, since most likely no completion item applies and it only distracts, obstructs the view, or is triggered accidentally. Thus, this patch prevents code completion from automatically getting triggered for numbers. It does not cover all cases (code completion will still show something for floating-point numbers that start with a dot, such as .1f) but should cover the most common cases. Manual completion with Ctrl+Space is still possible.

Test Plan: Tested manually.

Reviewers: KDevelop, mwolff

Reviewed By: KDevelop, mwolff

Subscribers: mwolff, brauch, kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D17915