[ORC] fix use-after-free detected by -Wreturn-stack-address

Summary:
llvm/lib/ExecutionEngine/Orc/Layer.cpp:53:12: warning: returning address of local temporary object [-Wreturn-stack-address]

In
```
StringRef IRMaterializationUnit::getName() const {
[...]
     return TSM.withModuleDo(
        [](const Module &M) { return M.getModuleIdentifier(); });
```
`getModuleIdentifier()` returns a `const std::string &`, but the implicit return type
of the lambda is `std::string` by value, and thus the returned `StringRef` refers
to a temporary `std::string`.

Detect by annotating `llvm::StringRef` with `[[gsl::Pointer]]`.

Reviewers: lhames, sgraenitz

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66440

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369306 91177308-0d34-0410-b5e6-96231b3b80d8

lib/ExecutionEngine/Orc/Layer.cpp

@@ -51,7 +51,7 @@ IRMaterializationUnit::IRMaterializationUnit(
 StringRef IRMaterializationUnit::getName() const {
   if (TSM)
     return TSM.withModuleDo(
-        [](const Module &M) { return M.getModuleIdentifier(); });
+        [](const Module &M) -> StringRef { return M.getModuleIdentifier(); });
   return "<null module>";
 }