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.

lld/ELF/ScriptLexer.cpp

@@ -64,7 +64,7 @@ size_t ScriptLexer::getColumnNumber() {
 }
 
 std::string ScriptLexer::getCurrentLocation() {
-  std::string filename = getCurrentMB().getBufferIdentifier();
+  std::string filename = std::string(getCurrentMB().getBufferIdentifier());
   return (filename + ":" + Twine(getLineNumber())).str();
 }