[llvm] Add StringMap::lookup() overload to take a default value and return a reference to the value to avoid a copy (PR #115469)

[Jakub Kuderski via llvm-commits]

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@@ -257,6 +257,15 @@ class LLVM_ALLOCATORHOLDER_EMPTYBASE StringMap
     return ValueTy();
   }
 
+  /// lookup - Return the entry for the specified key, or a default
+  /// provided value if no such entry exists.
+  const ValueTy &lookup(StringRef Key, const ValueTy& Value) const {
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kuhar wrote:

This seems like quite a footgun to me: the lifetime of the default value won't be extended beyond the lookup expression, whereas the looked up object will be kept alive by the map.

Consider this code:
```c++
StringMap<SmallVector<int>> foo;
const auto &ints = foo.lookup("X", SmallVector<int>{});
```

I think `ints` will be a dangling reference.

https://github.com/llvm/llvm-project/pull/115469