There was some discussion of joining forces in the early days of LLVM (ca. 2005 I believe), but for various reasons that did not go forward. Licensing is indeed a big issue: GCC has had the explicit goal of making proprietary plugins technically infeasible, which leads to avoidance of some modularization/decoupling opportunities.
They already do- for a long time before Clang was ready for prime time, the default compiler on OS X was GCC with an LLVM back end. I'm not sure if any of this ever got merged into mainline GCC (the work was done on a pre-GPLv3 fork of GCC), but the code is widely available per the terms of the GPL.
There was some discussion of joining forces in the early days of LLVM (ca. 2005 I believe), but for various reasons that did not go forward. Licensing is indeed a big issue: GCC has had the explicit goal of making proprietary plugins technically infeasible, which leads to avoidance of some modularization/decoupling opportunities.