GCC far far far predates LLVM. They do not share code.

The libsanitizer library is imported from the LLVM sources into the GCC source tree. There are probably more examples of code sharing. Both projects are quite large.

	▲	steve1977 2 hours ago \| parent [-]
		I would assume the code sharing is one-way only due to the licenses. I.e. a GPL project can use MIT licensed code, but not vice versa (AFAIK).