[clang] Add -Wmissing-designated-field-initializers (#81364)

#56628 changed the behavior of `-Wmissing-field-initializers`, which
introduces many new warnings in C++ code that uses partial designated
initializers. If such code is being built with `-Wextra -Werror`, this
change will break the build.

This PR adds a new flag that allows to disable these new warnings and
keep the old ones, as was suggested by @AaronBallman in the original
issue:
https://github.com/llvm/llvm-project/issues/56628#issuecomment-1761510850

Fixes  #68933