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/source/Utility/Log.cpp b/lldb/source/Utility/Log.cpp
index c8f32f4..4df82f2 100644
--- a/lldb/source/Utility/Log.cpp
+++ b/lldb/source/Utility/Log.cpp
@@ -138,7 +138,7 @@
 
   Stream << Content << "\n";
 
-  WriteMessage(FinalMessage.str());
+  WriteMessage(std::string(FinalMessage.str()));
 }
 
 // Printing of errors that are not fatal.