TargetMachine: Merge TargetMachine and LLVMTargetMachine

Merge LLVMTargetMachine into TargetMachine.

- There is no in-tree target anymore that just implements TargetMachine
  but not LLVMTargetMachine.
- It should still be possible to stub out all the various functions in
  case a target does not want to use lib/CodeGen
- This simplifies the code and avoids methods ending up in the wrong
  interface.

Differential Revision: https://reviews.llvm.org/D38489

llvm-svn: 315633