[Sema] Suppress additional warnings for C's zero initializer

[Sema] Suppress additional warnings for C's zero initializer

Summary:
D28148 relaxed some checks for assigning { 0 } to a structure for all C
standards, but it failed to handle structures with non-integer
subobjects. Relax -Wmissing-braces checks for such structures, and add
some additional tests.

This fixes PR39931.

Patch By: al3xtjames

Reviewed By: Lekensteyn

Differential Revision: https://reviews.llvm.org/D61838

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@366163 91177308-0d34-0410-b5e6-96231b3b80d8