Make llvm::StringRef to std::string conversions explicit.

This is how it should've been and brings it more in line with
std::string_view. There should be no functional change here.

This is mostly mechanical from a custom clang-tidy check, with a lot of
manual fixups. It uncovers a lot of minor inefficiencies.

This doesn't actually modify StringRef yet, I'll do that in a follow-up.
diff --git a/lldb/utils/TableGen/LLDBOptionDefEmitter.cpp b/lldb/utils/TableGen/LLDBOptionDefEmitter.cpp
index 6e73d0c5..ccf4827 100644
--- a/lldb/utils/TableGen/LLDBOptionDefEmitter.cpp
+++ b/lldb/utils/TableGen/LLDBOptionDefEmitter.cpp
@@ -55,18 +55,18 @@
     Required = Option->getValue("Required");
 
     // Add the full and short name for this option.
-    FullName = Option->getValueAsString("FullName");
-    ShortName = Option->getValueAsString("ShortName");
+    FullName = std::string(Option->getValueAsString("FullName"));
+    ShortName = std::string(Option->getValueAsString("ShortName"));
 
     if (auto A = Option->getValue("ArgType"))
       ArgType = A->getValue()->getAsUnquotedString();
     OptionalArg = Option->getValue("OptionalArg") != nullptr;
 
     if (Option->getValue("Validator"))
-      Validator = Option->getValueAsString("Validator");
+      Validator = std::string(Option->getValueAsString("Validator"));
 
     if (Option->getValue("ArgEnum"))
-      ArgEnum = Option->getValueAsString("ArgEnum");
+      ArgEnum = std::string(Option->getValueAsString("ArgEnum"));
 
     if (Option->getValue("Completions"))
       Completions = Option->getValueAsListOfStrings("Completions");