Fix clang assertion while building recent www/chromium

Merge commit c8227f06b335 from llvm git (by Arthur Eubanks):

[clang] Don't assert in EmitAggregateCopy on trivial_abi types

Fixes PR42961.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D97872

PR: 256721, 255570
Reported by: jbeich
MFC after: 3 days