By being permissively licensed, it was free from all the garbage and ideological baggage that GCC has, so they could use it in meaningful ways besides being just a compiler. Like using it to do analysis on code in the IDE.
That's because LLVM have those features, not because it has that license. A program designed to do analysis on code in the IDE will still do it even if the program is proprietary, which is the inevitable consequence of promoting permissive licenses.
