Make the -Wc++11-compat warnings ignored by default, so we don't break

valid C++98/03 code. However, add these warnings to -Wall, for those
who obviously already like clean code.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@142903 91177308-0d34-0410-b5e6-96231b3b80d8