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/llvm/lib/Analysis/VFABIDemangling.cpp b/llvm/lib/Analysis/VFABIDemangling.cpp
index a331b95..84cad1a 100644
--- a/llvm/lib/Analysis/VFABIDemangling.cpp
+++ b/llvm/lib/Analysis/VFABIDemangling.cpp
@@ -403,7 +403,7 @@
            "The global predicate must be the last parameter");
 
   const VFShape Shape({VF, IsScalable, Parameters});
-  return VFInfo({Shape, ScalarName, VectorName, ISA});
+  return VFInfo({Shape, std::string(ScalarName), std::string(VectorName), ISA});
 }
 
 VFParamKind VFABI::getVFParamKindFromString(const StringRef Token) {