[HLSL] Make load methods take const attribute (#193858)
Previously, HLSL shaders that declare a local resource const, cannot use
the `Load` method, since it is not attributed with const.
Load does not change a resource, it is a read-only operation, so the
method should be marked const.
This PR changes the methods so that they are marked const.
Fixes https://github.com/llvm/llvm-project/issues/192557
Assisted by: Github Copilot
