[LLDB][NFC] Remove redundant target/process checks in SBFrame (#153258)

This is a follow up to https://github.com/llvm/llvm-project/pull/152020,
continuing the removal of now-redundant `if(process && target)` checks.
Since this causes a diff in every line of the affected functions, this
commit also uses the opportunity to create some helper functions and
reduce nesting of the affected methods by rewriting all pre-condition
checks as early returns, while remaining strictly NFC.

This has exposed some odd behaviors:

1. `SBFrame::GetVariables` has a variable `num_produced` which is
clearly meant to be incremented on every iteration of the loop but it is
only incremented once, after the loop. So its value is always 0 or
   1. The variable now lives in `FetchVariablesUnlessInterrupted`.
2. `SBFrame::GetVariables` has an interruption mechanism for local
variables, but not for "recognized arguments". It's unclear if this is
by design or not, but it is now evident that there is a discrepancy
there.
3. In `SBFrame::EvaluateExpression` we only log some error paths, but
not all of them.

To stick to the strictly NFC nature of this patch, it does not address
any of these issues.