[stream_executor:host] Add HostExecutionEngine
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[stream_executor:host] Add HostExecutionEngine

This is a part of the ongoing effort https://github.com/openxla/xla/issues/7234
Add:
- RuntimeExecutionEngine class which compiles LLVM IR into kernel. This is a simplified copy-paste from runtime/execution_engine
- HostExecutionEngine virtual base class which contains JIT-compiled function.
- LlvmExecutionEngine, which uses two classes above to compile LLVM IR and store result.
- Dummy CppExecutionEngine for kernels written in C++ and precompiled together with XLA.
- HostKernelTest.LlvmAddition test which verifies only compilation (for now).
