@regehr "Although I can’t prove it, I like to think that these tools (and others like them) have helped the production compilers that developers use every day become more robust and solid."

I can't totally prove it either but if these tools are run regularly on the trunk of the compilers; they have found bugs earlier than doing a full distro build. At least for GCC.
For GCC, the runtime fuzzier testing has usually found bugs that were introduced in the last week or so which make it easier for a developer/reviewer just to fix it as that part of the code is fresh (usually).

(note LLVM insight might be harder due to bug/regression tracking is something which is lacking behind compared to GCC; that and the folks running the runtime fuzziers might not report reports upstream but only downstream from what i can tell).