[Sema] Fix -Wuninitialized for struct assignment from GNU C statement expression

Summary:
Do not automatically report self references of structs in statement expression
as warnings. Instead wait for uninitialized cfg analysis.
https://bugs.llvm.org/show_bug.cgi?id=42604

Reviewers: aaron.ballman, rsmith, nickdesaulniers

Reviewed By: aaron.ballman, nickdesaulniers

Subscribers: nathanchance, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64678

llvm-svn: 367134