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.

lldb/source/Plugins/ObjectFile/ELF/ObjectFileELF.cpp

@@ -1696,7 +1696,7 @@ class VMAddressProvider {
 
 public:
   VMAddressProvider(ObjectFile::Type Type, llvm::StringRef SegmentName)
-      : ObjectType(Type), SegmentName(SegmentName) {}
+      : ObjectType(Type), SegmentName(std::string(SegmentName)) {}
 
   std::string GetNextSegmentName() const {
     return llvm::formatv("{0}[{1}]", SegmentName, SegmentCount).str();