Move the C++11 ExtWarn for converting a string literal to 'char*' out of
-Wc++11-compat-deprecated-writable-strings. It's neither a C++11 compatibility
warning nor a deprecated feature, it's just ill-formed.

In passing, add that warning to -Wdeprecated, where it belongs.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206833 91177308-0d34-0410-b5e6-96231b3b80d8