[Sema] Diagnose default-initialization, destruction, and copying of
non-trivial C union types
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.
- 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.
Differential Revision: https://reviews.llvm.org/D63753