[clang] [UBSAN][NFC] add coverage for null and alignment checks for aggregates (PR #175023)

[Hubert Tong via cfe-commits]

https://github.com/hubert-reinterpretcast commented:

@vasu-the-sharma, the test matrix asked for in https://github.com/llvm/llvm-project/pull/164548#discussion_r2530103045 is necessary (to demonstrate that the fix strategy covers a broad number of cases and to reduce the risk of regression for any one case) due to:

1. the AST-based nature of the code generation,
2. the instrumentation covering one-past-the-end array indexing in addition to null pointer and alignment checks, and
3. differences between C and C++.

https://github.com/llvm/llvm-project/pull/175023