Frontc parse error due to `_Alignas(...)`

When (incrementally) analyzing commit https://github.com/facebook/zstd/commit/1c7d2c4dd561e557964b78c3757ee4cc745be445 of the zstd repository the following parse error is thrown:
```
zstd/lib/common/xxhash.h[1093:0-0] : syntax error
Parsing errorFatal error: exception Frontc.ParseError("Parse error")
```