Use StringRef instead of char* (NFC)

(cherry picked from commit 8b62429)

Author: adrian-prantl

lldb/include/lldb/Expression/UserExpression.h

@@ -266,10 +266,8 @@ class UserExpression : public Expression {
       0x1001; ///< ValueObject::GetError() returns this if there is no result
               /// from the expression.
 
-  const char *GetFixedText() {
-    if (m_fixed_text.empty())
-      return nullptr;
-    return m_fixed_text.c_str();
+  llvm::StringRef GetFixedText() {
+    return m_fixed_text;
   }
 
 protected:

lldb/source/Expression/UserExpression.cpp

@@ -262,9 +262,7 @@ UserExpression::Evaluate(ExecutionContext &exe_ctx,
   if (fixed_expression == nullptr)
     fixed_expression = &tmp_fixed_expression;
 
-  const char *fixed_text = user_expression_sp->GetFixedText();
-  if (fixed_text != nullptr)
-    fixed_expression->append(fixed_text);
+  *fixed_expression = user_expression_sp->GetFixedText().str();
 
   // If there is a fixed expression, try to parse it:
   if (!parse_success) {
@@ -293,8 +291,8 @@ UserExpression::Evaluate(ExecutionContext &exe_ctx,
         } else {
           // The fixed expression also didn't parse. Let's check for any new
           // Fix-Its we could try.
-          if (fixed_expression_sp->GetFixedText()) {
-            *fixed_expression = fixed_expression_sp->GetFixedText();
+          if (!fixed_expression_sp->GetFixedText().empty()) {
+            *fixed_expression = fixed_expression_sp->GetFixedText().str();
           } else {
             // Fixed expression didn't compile without a fixit, don't retry and
             // don't tell the user about it.