Revert r363326 "[libFuzzer] simplify the DFT trace collection using the new...

Revert r363326 "[libFuzzer] simplify the DFT trace collection using the new faster DFSan mode that traces up to 16 labels at a time and never runs out of labels."

It broke the Windows build:

C:\b\s\w\ir\cache\builder\src\third_party\llvm\compiler-rt\lib\fuzzer\FuzzerDataFlowTrace.cpp(243): error C3861: 'setenv': identifier not found

This also reverts the follow-up r363327.

llvm-svn: 363358