I’m only ever using a bleeding edge Clang trunk build myself. So when Fedora enabled use of the GCC abi_tag attribute in libstdc++ (i.e., switched the _GLIBCXX_USE_CXX11_ABI macro to 1 by default), it was only a matter of applying a set of (evolving) patches to Clang to keep things working.
However, others who wanted to use the Clang version included in Fedora were in for more of a problem. The typical symptom being that what you compiled with Clang fails to link against system libraries, most notably libstdc++.
For Fedora 23, we included a somewhat slapdash early version of the relevant Clang patches, but that didn’t survive the split-out of Clang from the overall LLVM package in Fedora 24. But now that the Clang upstream patches are deemed stable (and have been integrated into trunk towards Clang 3.9), I backported them to the Clang 3.8 in Fedora 24 (and rawhide).
And a lot of thanks to all of those who created these patches!