Fix standalone Lit discovery

[shin4141]

Fixes #432

Summary

Cling's standalone CMake configuration currently decides whether to enable tests using LLVM_LIT, but modern LLVM's AddLLVM test helpers execute LLVM_EXTERNAL_LIT / LLVM_DEFAULT_EXTERNAL_LIT.

That creates two problems:

• a valid explicit or LLVM-provided Lit executable can be ignored by Cling's availability gate;
• the fallback search only looks for an installed executable literally named lit.py, while current Lit installations commonly provide lit or LLVM's platform launcher.

This change keeps LLVM-provided Lit configuration authoritative, preserves source-tree Lit discovery, and only falls back to platform-appropriate installed executables when LLVM has not already supplied one. A fallback discovered by Cling is forwarded through LLVM_EXTERNAL_LIT so the same executable is used by AddLLVM.

If no valid Lit executable is available, standalone tests remain disabled as before.

Verification

I exercised standalone CMake configuration with isolated cases for:

• explicit LLVM_EXTERNAL_LIT
• LLVM-provided LLVM_DEFAULT_EXTERNAL_LIT
• source-tree utils/lit/lit.py
• installed llvm-lit
• installed lit
• native-Windows llvm-lit.py selection logic
• no Lit available

The precedence is:

1. explicit LLVM_EXTERNAL_LIT
2. LLVM-provided LLVM_DEFAULT_EXTERNAL_LIT
3. source-tree Lit
4. platform LLVM Lit executable
5. installed lit

A negative-control run restoring the old lit.py-only fallback reproduced the original failure: CMake configured successfully but disabled Cling tests when only a valid installed lit executable was available.

git diff --check also passes.

I did not run a full Cling build or check-cling locally because this environment does not contain a real built LLVM/Clang installation.