[Sema] Diagnose default-initialization, destruction, and copying of

[Sema] Diagnose default-initialization, destruction, and copying of
non-trivial C union types

This recommits r365985, which was reverted because it broke a few
projects using unions containing non-trivial ObjC pointer fields in
system headers. We now have a patch to fix the problem (see
https://reviews.llvm.org/D65256).

Original commit message:

This patch diagnoses uses of non-trivial C unions and structs/unions
containing non-trivial C unions in the following contexts, which require
default-initialization, destruction, or copying of the union objects,
instead of disallowing fields of non-trivial types in C unions, which is
what we currently do:

- function parameters.
- function returns.
- assignments.
- compound literals.
- block captures except capturing of `__block` variables by non-escaping blocks.
- local and global variable definitions.
- lvalue-to-rvalue conversions of volatile types.

See the discussion in https://reviews.llvm.org/D62988 for more background.

rdar://problem/50679094

Differential Revision: https://reviews.llvm.org/D63753

llvm-svn: 371275