Revert "[ExecutionEngine] Use std::function rather than a function pointer for...

Revert "[ExecutionEngine] Use std::function rather than a function pointer for the LazyFunctionCreator."

This reverts commit r241962, as it was breaking all ARM buildbots.

It also reverts the two subsequent related commits:

r241974: "[ExecutionEngine] Add a static cast to the unittest for r241962 to suppress a warning."

r241973: "[ExecutionEngine] Remove cruft and fix a couple of warnings in the test case for r241962."
llvm-svn: 241983