[llvm] Stop using __attribute__((retain)) in GCC builds (PR #133793)

[Fangrui Song via llvm-commits]

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@@ -231,8 +231,15 @@
 #endif
 
 #if __has_attribute(retain)
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MaskRay wrote:

perhaps `defined(__clang__) && __has_attribute(retain)` and just don't check `__GNUC__`

https://github.com/llvm/llvm-project/pull/133793