[unittests/CodeGen] Remove unique_ptr from the result of createTargetMachine

The object contained in unique_ptr will be automatically deleted at the end of
the current scope. In createMachineFunction,

  auto TM = createTargetMachine();

creates a TM contained in unique_ptr, a reference of the TM is stored in a
MachineFunction object, but at the end of the function, the TM is deleted, so
later access to the TM(and contained STI, TRI ...) through MachineFunction
object is invalid.

So we should not use unique_ptr<BogusTargetMachine> in functions
createMachineFunction and createTargetMachine.

Differential Revision: https://reviews.llvm.org/D131790