[LLDB][NativePDB] Fix a crash when S_DEFRANGE_SUBFIELD_REGISTER descirbes a simple type

When a variable is simple type and has 64 bits, the debug info may look like the following when targeting 32bit windows. The variable's content is split into two 32bits registers.
```
480 | S_LOCAL [size = 12] `x`
      type=0x0013 (__int64), flags = param
492 | S_DEFRANGE_SUBFIELD_REGISTER [size = 20]
      register = EAX, may have no name = true, offset in parent = 0
      range = [0001:0073,+7), gaps = []
512 | S_DEFRANGE_SUBFIELD_REGISTER [size = 20]
      register = ECX, may have no name = true, offset in parent = 4
      range = [0001:0073,+7), gaps = []
```

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D122943