[clang] Change the condition of unnecessary packed warning

Summary:
Change the condition of this unnecessary packed warning. The packed is unnecessary when
1. the alignment of the struct/class won't alter.
2. the size is unchanged.
3. 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

llvm-svn: 309750