coverage: clean possible mainline runs before collecting coverage
coverage: clean possible mainline runs before collecting coverage

If the Makefile has already been generated, things might have been
configured and built using clang, which also means ASan symbols that
GCC will not like will be around. Clean the repository with distclean
before running if signs of previous compilations are present.

curl/curl-fuzzer,curl/curl-fuzzer,curl/curl-fuzzer,curl/curl-fuzzer