Make adjustments for LLVM-19

[linehill]

• Fixed LLVM passes for LLVM-19.

• Changed chipStar configuration to prefer llvm-spirv with version suffix which matches to the chosen LLVM (e.g. llvm-spirv-19).

• Update README and scripts/configure_llvm.sh.

• New in clang-19: clang's SPIR-V and HIPSPV toolchain picks llvm-spirv-XX (when found in PATH or installed alongside the clang binary) where XX matches to the clang's major version. This should resolve the llvm-spirv issue reported in #824.

TODOs before the PR is ready for landing:

• [X] Make chipStar forks for LLVM 19.
• [x] Wait until https://github.com/KhronosGroup/SPIRV-LLVM-Translator/pull/2657 lands and is backported to llvm_release_190.