[LLDB] Add an llvm::Optional version of GetRegisterInfo

We have some 500 ish uses of the bool plus ref version
so changing them all at once isn't a great idea.

This adds an overload that doesn't take a RegisterInfo&
and returns an optional.

Once I'm done switching all the existing callers I'll
remove the original function.

Benefits of optional over bool plus ref:
* The intent of the function is clear from the prototype.
* It's harder to forget to check if the return is valid,
  and if you do you'll get an assert.
* You don't hide ununsed variables, which happens because
  passing by ref marks a variable used.
* You can't forget to reset the RegisterInfo in between
  calls.

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D134536