Pass NO_MISSING_FIELD_INITIALIZERS to Compiler Flags

configure checks whether -Wno-missing-field-initializers is a valid
compiler flag but it was never actually used in Makefile.rules.
Enable it to avoid some ridiculous warnings from gcc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172870 91177308-0d34-0410-b5e6-96231b3b80d8