[PassBuilder] Support O0 in default pipelines

The default and pre-link pipeline builders currently require you to
call a separate method for optimization level O0, even though they
have perfectly well-defined O0 optimization pipelines.

Accept O0 optimization level and call buildO0DefaultPipeline()
internally, so all consumers don't need to repeat this.

Differential Revision: https://reviews.llvm.org/D146200