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

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."

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241983 91177308-0d34-0410-b5e6-96231b3b80d8