This reapplies bb27e4564355243e479cab40885d6e0f7f640572 (SimpleRemoteEPC support) and 2269a941a450a0d395161cfb792be58870b2875b (#include <mutex> fix) with further fixes to support building with LLVM_ENABLE_THREADS=Off.
This reapplies bb27e4564355243e479cab40885d6e0f7f640572 (SimpleRemoteEPC support) and 2269a941a450a0d395161cfb792be58870b2875b (#include <mutex> fix) with further fixes to support building with LLVM_ENABLE_THREADS=Off.