[libFuzzer] Don't complain about lack of interesting inputs when -runs=0.

Summary:
The most common usecase for -runs=0 is for generating code coverage
over some corpus. Coverage reports based on sancov are about to be deprecated,
which means some external coverage solution will be used, e.g. Clang source
based code coverage, which does not use any sancov instrumentations and thus
libFuzzer would consider any input to be not interesting in that case.

Reviewers: kcc

Reviewed By: kcc

Subscribers: alex, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D47271

llvm-svn: 333116