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/utils/TableGen/CodeGenMapTable.cpp b/llvm/utils/TableGen/CodeGenMapTable.cpp
index 793bb61..baca076 100644
--- a/llvm/utils/TableGen/CodeGenMapTable.cpp
+++ b/llvm/utils/TableGen/CodeGenMapTable.cpp
@@ -98,7 +98,7 @@
 
 public:
   InstrMap(Record* MapRec) {
-    Name = MapRec->getName();
+    Name = std::string(MapRec->getName());
 
     // FilterClass - It's used to reduce the search space only to the
     // instructions that define the kind of relationship modeled by