Explicitly derive alloca AS from DL (#45900)
* Explicitly derive alloca AS from DL
Fixes issues around usage of the AMDGPU LLVM target by GPUCompiler
* Use alloca AS for sret pointer type
* Insert addrspacecast in stringConstPtr
* text/llvmpasses: Test non-0 alloca addrspace
Co-authored-by: Collin Warner <collinw@mit.edu>
