[IR2Vec] Simplifying creation of Embedder (#143999)

This change simplifies the API by removing the error handling complexity. 

- Changed `Embedder::create()` to return `std::unique_ptr<Embedder>` directly instead of `Expected<std::unique_ptr<Embedder>>`
- Updated documentation and tests to reflect the new API
- Added death test for invalid IR2Vec kind in debug mode
- In release mode, simply returns nullptr for invalid kinds instead of creating an error

(Tracking issue - #141817)

llvm/docs/MLGO.rst

@@ -488,14 +488,9 @@ embeddings can be computed and accessed via an ``ir2vec::Embedder`` instance.
 
       // Assuming F is an llvm::Function&
       // For example, using IR2VecKind::Symbolic:
-      Expected<std::unique_ptr<ir2vec::Embedder>> EmbOrErr =
+      std::unique_ptr<ir2vec::Embedder> Emb =
           ir2vec::Embedder::create(IR2VecKind::Symbolic, F, Vocabulary);
 
-      if (auto Err = EmbOrErr.takeError()) {
-        // Handle error in embedder creation
-        return;
-      }
-      std::unique_ptr<ir2vec::Embedder> Emb = std::move(*EmbOrErr);
 
 3. **Compute and Access Embeddings**:
    Call ``getFunctionVector()`` to get the embedding for the function.