[lldb][test] TestCompactVectors.py: skip for older Clang
Was failing on the Clang 17 green dragon matrix bot:
```
/Applications/Xcode_26.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX26.2.sdk/System/Library/Frameworks/vecLib.framework/Headers/Sparse/SolveImplementationTyped.h:678:9: error: use of undeclared identifier '__builtin_verbose_trap'
07:28:37 678 | __builtin_verbose_trap("unsupported", "LU factorization is not supported on this OS version");
07:28:37 | ^
07:28:37 /Applications/Xcode_26.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX26.2.sdk/System/Library/Frameworks/vecLib.framework/Headers/Sparse/SolveImplementationTyped.h:888:95: error: unrecognized platform name visionOS
07:28:37 888 | if(__builtin_available(macOS 15.5, macCatalyst 18.5, iOS 18.5, watchOS 11.5, tvOS 18.5, visionOS 2.5, *)) {
07:28:37 |
```
d49060a72d