[clang] Change the condition of unnecessary packed warning
Change the condition of this unnecessary packed warning. The packed is unnecessary when
- the alignment of the struct/class won't alter.
- the size is unchanged.
- the offset of each field is the same.
Remove all field-level warning.
Reviewers: chh, akyrtzi, rtrieu
Reviewed By: chh
Subscribers: rsmith, srhines, cfe-commits, xazax.hun
Differential Revision: https://reviews.llvm.org/D34114