Fix use-after-free

[mgehre]

Summary

The warning

lldb/source/Core/FormatEntity.cpp:2350:25: warning: object backing the pointer will be destroyed at the end of the full-expression [-Wdangling]

is emitted after annotating llvm::StringRef with [[gsl::Pointer]].

The reason is that in

size_t FormatEntity::AutoComplete(CompletionRequest &request) {
 llvm::StringRef str = request.GetCursorArgumentPrefix().str();

the function GetCursorArgumentPrefix() returns a StringRef, and StringRef::str() returns
a temporary std::string.

lldb/trunk/source/Core/FormatEntity.cpp

for (size_t i = 0; i < n; ++i) {
                        match_prefix.size()) == 0)
         matches.AppendString(
             MakeMatch(prefix, def->children[i].name + match_prefix.size()));
     }
   }
 }
 
 size_t FormatEntity::AutoComplete(CompletionRequest &request) {
-  llvm::StringRef str = request.GetCursorArgumentPrefix().str();
+  llvm::StringRef str = request.GetCursorArgumentPrefix();
 
   request.SetWordComplete(false);
 
   const size_t dollar_pos = str.rfind('$');
   if (dollar_pos == llvm::StringRef::npos)
     return 0;
 
   // Hitting TAB after $ at the end of the string add a "{"