[lldb][nfc] Fix missing move operations and constness of methods (#142052)

This PR adds the missing move operators for VariableList: this class is
just a wrapper around a vector, so it can use the default move
operations. Subsequent patches will want to return VariableLists from
functions, so the move operation is required (the copy constructors are
deleted).

It also fixes constness for a method in DiagnosticManager returning its
list of diagnostics. Previously, the method always returned a const
list, even though the method was not const itself. Subsequent patches
will make use of the ability to mutate the diagnostics.

Author: felipepiovezan

lldb/include/lldb/Expression/DiagnosticManager.h

@@ -107,7 +107,8 @@ class DiagnosticManager {
     m_fixed_expression.clear();
   }
 
-  const DiagnosticList &Diagnostics() { return m_diagnostics; }
+  const DiagnosticList &Diagnostics() const { return m_diagnostics; }
+  DiagnosticList &Diagnostics() { return m_diagnostics; }
 
   bool HasFixIts() const {
     return llvm::any_of(m_diagnostics,

lldb/include/lldb/Symbol/VariableList.h

@@ -24,6 +24,9 @@ class VariableList {
   VariableList();
   virtual ~VariableList();
 
+  VariableList(VariableList &&) = default;
+  VariableList &operator=(VariableList &&) = default;
+
   void AddVariable(const lldb::VariableSP &var_sp);
 
   bool AddVariableIfUnique(const lldb::VariableSP &var_sp);