[clang] [llvm] [Support] Add getAllocTokenModeAsString() helper (PR #169650)

[Marco Elver via cfe-commits]

melver wrote:

> Did we factor this out b/c we are going to be reusing this in more than one place?

Yes:
```
% git grep getAllocTokenModeAsString
clang/lib/CodeGen/CodeGenModule.cpp:    StringRef S = llvm::getAllocTokenModeAsString(*LangOpts.AllocTokenMode);
clang/lib/Frontend/CompilerInvocation.cpp:    StringRef S = llvm::getAllocTokenModeAsString(*Opts.AllocTokenMode);
```

https://github.com/llvm/llvm-project/pull/169650