[DirectX] TargetExtType not being lowered properly in DXILOpLowering

[spall]

A TargetExtTy ( target("dx.RawBuffer", <4 x float>, 0, 0)) makes it into the DXILBitcodeWriter.
See for the error: https://godbolt.org/z/zT4coTbhW

Minimized: https://godbolt.org/z/zY9nEh51h

[damyanp]

@bogner to find the work the @hekota is planning to do and dupe as appropriate.

[bogner]

This falls under the "locally scoped resources" work. I'm going to call this a duplicate of #134154, though that may only be the first step in what needs to be done for this to work fully.