Fix string-literal to char* conversion in overload resolution for C++11

Fix string-literal to char* conversion in overload resolution for C++11

String literal to char* conversion is deprecated in C++03, and is removed in
C++11. We still accept this conversion in C++11 mode as an extension, if we find
it in the best viable function.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199513 91177308-0d34-0410-b5e6-96231b3b80d8