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/unittests/TextAPI/TextStubV1Tests.cpp b/llvm/unittests/TextAPI/TextStubV1Tests.cpp
index 4472b1c..68baac1 100644
--- a/llvm/unittests/TextAPI/TextStubV1Tests.cpp
+++ b/llvm/unittests/TextAPI/TextStubV1Tests.cpp
@@ -113,9 +113,9 @@
   for (const auto *Sym : File->symbols()) {
     EXPECT_FALSE(Sym->isWeakReferenced());
     EXPECT_FALSE(Sym->isUndefined());
-    Exports.emplace_back(ExportedSymbol{Sym->getKind(), Sym->getName(),
-                                        Sym->isWeakDefined(),
-                                        Sym->isThreadLocalValue()});
+    Exports.emplace_back(
+        ExportedSymbol{Sym->getKind(), std::string(Sym->getName()),
+                       Sym->isWeakDefined(), Sym->isThreadLocalValue()});
   }
   llvm::sort(Exports.begin(), Exports.end());