TargetPassConfig: Keep a reference to an LLVMTargetMachine; NFC

TargetPassConfig: Keep a reference to an LLVMTargetMachine; NFC

TargetPassConfig is not useful for targets that do not use the CodeGen
library, so we may just as well store a pointer to an
LLVMTargetMachine instead of just to a TargetMachine.

While at it, also change the constructor to take a reference instead of a
pointer as the TM must not be nullptr.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304247 91177308-0d34-0410-b5e6-96231b3b80d8