fix(analysis): reject shielded types with transient storage

Transient storage is a separate storage space that requires special
privacy considerations for traces. Reject shielded transient variables
at the declaration level with DeclarationError 9826.

The previous codegen fix (TSTORE/TLOAD for transient shielded) remains
as defense-in-depth but is now unreachable.

Author: cdrappi

libsolidity/analysis/DeclarationTypeChecker.cpp

@@ -508,6 +508,13 @@ void DeclarationTypeChecker::endVisit(VariableDeclaration const& _variable)
 						"Initialization of transient storage state variables is not supported."
 					);
 
+				if (_variable.typeName().annotation().type->isShielded())
+					m_errorReporter.declarationError(
+						9826_error,
+						_variable.location(),
+						"Shielded types cannot be used with transient storage."
+					);
+
 				typeLoc = DataLocation::Transient;
 				break;
 			default: