[lldb] Remove ProcessStructReader from NSStringSummaryProvider (NFC)

Simplify getting the length of `NSPathStore2` strings.

`NSStringSummaryProvider` uses a single field from `NSPathStore2` instances,
its first ivar: `_lengthAndRefCount`. This change uses
`GetSyntheticChildAtOffset` to replace the use of `ProcessStructReader`, and
removes the hard coded `CompilerType` definition of `NSPathStore2`.

Differential Revision: https://reviews.llvm.org/D116461