Factor out bolt project dependency from Clang-BOLT usage.
- user-specified BOLT: LLVM_BOLT and MERGE_FDATA variables,
- BOLT from system path (with CMake's find_program),
- built from bolt project (the only previously supported way).
Customize BOLT-PGO.cmake cache file to build BOLT in first-stage build, and
passthrough and use it in the final clang-bolt invocation (on top of PGO binary).
This avoids building PGO'd BOLT (first instrumented and then optimized), cutting
the build time.