Turn off missing field initializer warnings for gcc
    
gcc produces false positives for empty braces so turning the warning off.
Instead, turning the warning on for clang so proper warnings aren't missed.
    
Reviewers: dblaikie, chandlerc


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174073 91177308-0d34-0410-b5e6-96231b3b80d8