Sema: Inherit the flexible array property from struct fields

A record which contains a flexible array member is itself a flexible
array member.  A struct which contains such a record should also
consider itself to be a flexible array member.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218378 91177308-0d34-0410-b5e6-96231b3b80d8