I am not considering 3rd party patents, but Apple's. If Apple includes some patented tech into a contribution to LLVM or Clang, what guarantees I have they will not sue non-Apple downstream users?
Then I suppose the community can fork from the revision prior to where that patented technique landed in the tree, just as with any other OSI-approved license.
Depends on one's perspective. Not long ago the meme de jour was that open-source transparency & fork-ability was the ultimate protection to corporate evils.
By the standard you're setting, the only safe compiler would be one you wrote yourself and revealed to no one. What compilers do you use?