Don't accept qualified 'int' main return types in C++ or standard C mode

C++ [basic.start.main]p1: "It shall have a return type of type int"

ISO C is also clear about this, so only accept 'int' with qualifiers in GNUMode
C.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212171 91177308-0d34-0410-b5e6-96231b3b80d8