"but if it hadn't been for gplv3 do you really maintain that apple wouldn't have stuck with it as a frontend for longer? "
Yes. Absolutely. It's been mentioned numerous times at conferences and other in-person meetings.
Apple did not write clang because of GPLv3. They wrote clang because they needed something that was
1. Faster than GCC 2. Offered better diagnostics 3. Could offer code completion and indexing for XCode.
So I'll concede that it likely wasn't a direct cause of the move to and support of llvm, clang didn't come along until later -- and after it must have been clear that gcc would eventually be gplv3 licensed.
God knows GCC's codebase is a rats nest that few people really want to work with, but if it hadn't been for gplv3 do you really maintain that apple wouldn't have stuck with it as a frontend for longer? The early releases of clang (and gcc+llvm before it) were problematic for a lot of mac developers at the time, after all.