[llvm] [NFC][LLVM] Fix some `llvm` namespace usage in Bitcode writer (PR #141709)

[Teresa Johnson via llvm-commits]

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@@ -60,9 +60,6 @@
 #include <type_traits>
 #include <utility>
 #include <vector>
-namespace llvm {
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teresajohnson wrote:

This one is unrelated to the other changes and can be split into a separate PR and committed. There is a use of Argument in this file, but presumably it is being pulled in via other headers, so no need for this forward def.

https://github.com/llvm/llvm-project/pull/141709