cmake: Invent add_llvm_fuzzer to set up fuzzer targets

This moves the cmake configuration for fuzzers in LLVM to a new macro,
add_llvm_fuzzer. This will make it easier to keep things consistent
while implementing llvm.org/pr34314.

I've also made a couple of minor functional changes here:

- the fuzzers now use add_llvm_executable rather than add_llvm_tool.
  This means they won't create install targets and stuff like that,
  because those made little sense for these fuzzers.
- I've grouped these under "Fuzzers" rather than in with "Tools" for
  people who build with IDEs.

llvm-svn: 312200