[lldb] Improve maintainability and readability for ValueObject methods (#75865)

As I worked through changes to another PR
(https://github.com/llvm/llvm-project/pull/74912), I couldn't help but
rewrite a few methods for readability, maintainability, and possibly
some behavior correctness too.

1. Exiting early instead of nested `if`-statements, which:
	- Reduces indentation levels for all subsequent lines
	- Treats missing pre-conditions similar to an error
- Clearly indicates that the full length of the method is the "happy
path".
2. Explicitly return empty Value Object shared pointers for those error
(like) situations, which
- Reduces the time it takes a maintainer to figure out what the method
actually returns based on those conditions.

3. Converting a mix of `if` and `if`-`else`-statements around an enum
into one `switch` statement, which:
	- Consolidates the former branching logic
	- Lets the compiler warn you of a (future) missing enum case
- This one may actually change behavior slightly, because what was an
early test for one enum case, now happens later on in the `switch`.

4. Consolidating near-identical, "copy-pasta" logic into one place,
which:
	- Separates the common code to the diverging paths.
	- Highlights the differences between the code paths.



rdar://119833526

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