[lldb/Fuzzer] Add fuzzer for expression evaluator

This commit adds a fuzzer for LLDB's expression evaluator.
The fuzzer takes a different approach than the current fuzzers
present, and uses an approach that is currently being used for
clang fuzzers.

Instead of fuzzing the evaluator with randomly mutated
characters, protobufs are used to generate a subset of C++. This
is then converted to valid C++ code and sent to the expression
evaluator. In addition, libprotobuf_mutator is used to mutate
the fuzzer's inputs from valid C++ code to valid C++ code, rather
than mutating from valid code to total nonsense.

Differential revision: https://reviews.llvm.org/D129377