[lldb] upgrade HandleFrameFormatVariable callees to llvm::Expected (#144731)

Upgrade the callees of `HandleFrameFormatVariable`
(`GetDemangledTemplateArguments`, etc), to return a `llvm::Expected`
instead of an `std::optional`.

This patch also bundles the logic of validating the demangled name and
information into a single reusable function to reduce code duplication.

Author: charles-zablit

lldb/include/lldb/Core/DemangledNameInfo.h

@@ -71,27 +71,28 @@ struct DemangledNameInfo {
 
   /// Returns \c true if this object holds a valid basename range.
   bool hasBasename() const {
+    // A function always has a name.
     return BasenameRange.second > BasenameRange.first;
   }
 
   /// Returns \c true if this object holds a valid scope range.
-  bool hasScope() const { return ScopeRange.second > ScopeRange.first; }
+  bool hasScope() const { return ScopeRange.second >= ScopeRange.first; }
 
   /// Returns \c true if this object holds a valid arguments range.
   bool hasArguments() const {
-    return ArgumentsRange.second > ArgumentsRange.first;
+    return ArgumentsRange.second >= ArgumentsRange.first;
   }
 
   /// Returns \c true if this object holds a valid qualifiers range.
   bool hasQualifiers() const {
-    return QualifiersRange.second > QualifiersRange.first;
+    return QualifiersRange.second >= QualifiersRange.first;
   }
 
   /// Returns \c true if this object holds a valid prefix range.
-  bool hasPrefix() const { return PrefixRange.second > PrefixRange.first; }
+  bool hasPrefix() const { return PrefixRange.second >= PrefixRange.first; }
 
   /// Returns \c true if this object holds a valid suffix range.
-  bool hasSuffix() const { return SuffixRange.second > SuffixRange.first; }
+  bool hasSuffix() const { return SuffixRange.second >= SuffixRange.first; }
 };
 
 /// An OutputBuffer which keeps a record of where certain parts of a