There's been many KDevelop Clang improvements landing recently and its quality is surpassing KDevelop's own C++ plug-in that has its own C++ parser and other own implementations. Milian Wolff of KDE explained about Clang for KDevelop as "the magic bullet which we waited for, and which did not exist back when KDevelop 4.0 was started initially."
Using Clang with KDevelop instead of its own in-house C++ language support leads to a great code reduction: ~50k lines of code for its own solution versus ~2600 lines of code for hooking into Clang. The Clang parser is also much faster than KDevelop's own C++ parser with greater parallelism. KDevelop can also more easily support new C++ features seamlessly as they are added to Clang. Whether Qt Creator will follow KDevelop in this Clang route is still being determined.
Those curious about this KDevelop Clang support for improving its C++ language support in the open-source KDE IDE can be found via this blog post. Clang's library-based design and greater flexibility and modularity than GCC and other compilers also makes it easy for automatic code reformatting, greater IDE integration, support for rewriting C++ code in more modern standards, and other interesting possibilities.