[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

llvm-svn: 369306