Use -std=c++11 when no compilation database is provided

Allow the migrator to be used without specifing --. If neither -- nor -p is
provided and no compilation database can be detecteded from the first source
file path then -std=c++11 is added as the only compiler argument.


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@188533 91177308-0d34-0410-b5e6-96231b3b80d8