asan: replace used python in mlir lit.cfg with shim script

[philippjh]

This PR exchanges the used Python executable inside the MLIR LIT test suite, when the build is configured for utilizing ASan. The executable is replaced with a shim script, which preloads the necessary ASan shared object file as it did before, but additionally exhibits two benefits:

1. Provide full support for LLVM's test utils like not, which are prepended to the original statement containing the LD_PRELOAD env definition. Having environment definitions in the middle of a command line is syntactically illegal.
2. Mitigate issues with LIT's internal shell that puts single quotes around the environment definition, which leads to malformed command lines: LD_PRELOAD=$(/usr/bin/clang++-17' '-print-file-name=libclang_rt.asan-x86_64.so)' python (...)
   • The problem might be circumvented by disabling the LIT's internal shell through updating the environment with LIT_USE_INTERNAL_SHELL=0, but might not be desired for compatibility/stability reasons.