[llvm] [NFC][LLVM] Namespace cleanup in MetadataLoader.cpp (PR #157595)

[Rahul Joshi via llvm-commits]

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@@ -58,9 +58,6 @@
 #include <tuple>
 #include <utility>
 #include <vector>
-namespace llvm {
-class Argument;
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jurahul wrote:

You're right. The use of `Argument` is in `isa_and_nonnull` and I think it does not work with a simple forward decl as it calls Argument::classof()`. Today this works because Function.h includes Argument.h but following IWYU we should include Argument.h explicitly.

https://github.com/llvm/llvm-project/pull/157595