[LLDB][NFC] Remove redundant target/process checks in SBFrame

This is a follow up to llvm#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.

Author: felipepiovezan

lldb/include/lldb/API/SBFrame.h

@@ -13,6 +13,8 @@
 #include "lldb/API/SBValueList.h"
 
 namespace lldb_private {
+class StackFrame;
+class Debugger;
 namespace python {
 class SWIGBridge;
 }
@@ -237,6 +239,23 @@ class LLDB_API SBFrame {
   /// not currently stopped.
   static SBValue CreateProcessIsRunningExprEvalError();
 
+  enum WasInterrupted { Yes, No };
+
+  /// Populates `value_list` with the variables from `frame` according to
+  /// `options`. This method checks whether the Debugger received an interrupt
+  /// before processing every variable, returning `WasInterrupted::yes` in that
+  /// case.
+  static WasInterrupted FetchVariablesUnlessInterrupted(
+      const lldb::SBVariablesOptions &options, lldb_private::StackFrame &frame,
+      SBValueList &value_list, lldb_private::Debugger &dbg);
+
+  /// Populates `value_list` with recognized arguments of `frame` according to
+  /// `options`.
+  static void FetchRecognizedArguments(const SBVariablesOptions &options,
+                                       lldb_private::StackFrame &frame,
+                                       SBTarget target,
+                                       SBValueList &value_list);
+
   lldb::ExecutionContextRefSP m_opaque_sp;
 };