Introduces LLVM intrinsic `llvm.dx.resource.getdimensions.x` and its lowering to DXIL op `op.dx.getDimensions`.
The intrinsic will be used to implement `GetDimension` for buffers. The lowering is using `undef` value since it is required by the DXIL format which is based on LLVM 3.7.
Proposal update: https://github.com/llvm/wg-hlsl/pull/350Closes#112982
