clang: Also detect Clang builtin dirs at runtime

Summary:
Changes:

• Add a helper function retrieving the Clang version at runtime
• Auto-detect bundled copy of Clang builtin headers on Windows
• Print lots more debug output during startup

TODO: Wondering if we want to add more code detecting this directory at
runtime on other platforms, too. But for now, only Windows is
interesting for us.

CCBUG: 393779

Reviewers: kossebau, aaronpuchert, brauch

Reviewed By: brauch

Subscribers: brauch, kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D15955