Update LLVM version (#5415)

Fixes a compile failure with the new version, essentially:

```
external/+llvm_project+llvm-project/llvm/include/llvm/Support/FormatVariadicDetails.h:157:1: error: implicit instantiation of undefined template 'llvm::support::detail::missing_format_adapter<clang::LookupResultKind>'
```

toolchain/check/import_cpp.cpp

@@ -629,7 +629,7 @@ auto ImportNameFromCpp(Context& context, SemIR::LocId loc_id,
     context.TODO(loc_id,
                  llvm::formatv("Unsupported: Lookup succeeded but couldn't "
                                "find a single result; LookupResultKind: {0}",
-                               lookup->getResultKind())
+                               static_cast<int>(lookup->getResultKind()))
                      .str());
     return SemIR::ErrorInst::InstId;
   }