[libFuzzer] Decrease stack usage in unit tests

Summary: With 3 Dictionary objects, each containing space of ~16k DictionaryEntry objects, the MutationDispatcher object is fairly memory heavy.  On platforms with a lower default stack size, this can cause panics in FuzzerUnittest as those tests stack-allocate the MutationDispatcher.  This may be especially problematic for platforms that do not (yet) have a way to programmatically change their stack size, aside from link-time flags.  In general, it seems more prudent to use the heap for an object of this size.

Reviewers: kcc, morehouse

Reviewed By: kcc

Differential Revision: https://reviews.llvm.org/D40926

llvm-svn: 319988