Safer casting in ClangExpressionParser code completion

[teemperor]

Summary

Makes the code a bit safer in the unlikely situation that we don't get a ClangUserExpression
when doing code completion.

[labath]

lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp

860–862

These two lines are equivalent to auto *llvm_expr = cast<ClangUserExpression>(&m_expr);

[teemperor]

• Changed to llvm::cast (even though the assert message is a bit generic now).

lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp

bool ClangExpressionParser::Complete(CompletionRequest &request, unsigned line,
                                      unsigned pos, unsigned typed_pos) {
   DiagnosticManager mgr;
   // We need the raw user expression here because that's what the CodeComplete
   // class uses to provide completion suggestions.
   // However, the `Text` method only gives us the transformed expression here.
   // To actually get the raw user input here, we have to cast our expression to
   // the LLVMUserExpression which exposes the right API. This should never fail
   // as we always have a ClangUserExpression whenever we call this.
-  LLVMUserExpression &llvm_expr = *static_cast<LLVMUserExpression *>(&m_expr);
-  CodeComplete CC(request, m_compiler->getLangOpts(), llvm_expr.GetUserText(),
+  ClangUserExpression *llvm_expr = cast<ClangUserExpression>(&m_expr);
+  CodeComplete CC(request, m_compiler->getLangOpts(), llvm_expr->GetUserText(),
                   typed_pos);
   // We don't need a code generator for parsing.
   m_code_generator.reset();
   // Start parsing the expression with our custom code completion consumer.
   ParseInternal(mgr, &CC, line, pos);
   return true;
 }
 
 unsigned ClangExpressionParser::Parse(DiagnosticManager &diagnostic_manager) {