[lld] Fix undefined behavior with misaligned SHT_GROUP section. (#180848) read32() allows misaligned values, but a `uint32_t &` must be properly aligned even if it isn't directly read. ubsan detects this. To fix the issue, replace the `uint32_t &` with a value that doesn't require alignment. Also added an assertion to catch similar misuse of getDataAs(). (Alternatively, we could make the input validation more strict, and reject files with a misaligned SHT_GROUP, but I don't see any obvious reason to require that.)
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