[HLSL] Fix resource wrapper declaration (#129100)
The resource wrapper should have internal linkage because it contains a
handle to the global resource, and it not the actual global.
Makeing this changed exposed that we were zeroinitializing the resouce,
which is a problem. The handle cannot be zeroinitialized. This is
changed to use poison instead.
Fixes https://github.com/llvm/llvm-project/issues/122767.
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Co-authored-by: Helena Kotas <hekotas@microsoft.com>
