1. Declare a virtual function getPointerToNamedFunction() in JITMemoryManager
2. Move the implementation of getPointerToNamedFunction() form JIT/MCJIT to DefaultJITMemoryManager.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153205 91177308-0d34-0410-b5e6-96231b3b80d8