[lldb][NFCI] Minor cleanups to StructuredData::GetObjectForDotSeparatedPath

[lldb][NFCI] Minor cleanups to StructuredData::GetObjectForDotSeparatedPath

This accomplishes a few minor things:
- Removed unnecessary uses of `this->`
- Removed an unnecessary std::string allocation.
- Removed some nesting to improve readability using early returns where
  it makes sense.
- Replaced `strtoul` with `llvm::to_integer` which avoids another
  std::string allocation.
- Removed braces from single statement conditions, removed
  else-after-returns.

Differential Revision: https://reviews.llvm.org/D154534