[GWP-ASan] Build stack_trace_compressor_fuzzer.

Summary:
Flips the switch to build stack_trace_compressor_fuzzer. This was recently
temporarily disabled in rL369079 as it was breaking the sanitizer buildbots.

My diagnosis of the problem is that on clang-only bootstrap builds, we build
gwp_asan before libfuzzer. This causes a discrepancy when the clang driver
attempts to link libclang_rt.fuzzer* as CMake doesn't see a dependency there.

I've (hopefully) fixed the issue by adding a direct dependency for the fuzz
target so CMake can resolve the build order properly. As part of this, the
libFuzzer 'fuzzer' target has to be discovered before the declaration of the
fuzz target.

pcc@ for mild review + notification as buildcop.

Reviewers: pcc

Reviewed By: pcc

Subscribers: mgorny, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D66494

llvm-svn: 369551