[gn build] Add fuzzers in llvm/tools that are needed for check-llvm

Also add a fuzzer() template for defining fuzzers that's similar to
add_llvm_fuzzer in the CMake build, and a build file for dependency
llvm/lib/FuzzMutate.

Also make `assert(defined(...` error strings a bit more self-consistent.

Differential Revision: https://reviews.llvm.org/D56194

llvm-svn: 350238