Enforce restrictions that 'main' is not allowed to be deleted, or to be used by
the program, in C++. (We allow the latter as an extension, since we've always
permitted it, and GCC does the same, and our supported C++ ABIs don't do
anything special in main.)


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199782 91177308-0d34-0410-b5e6-96231b3b80d8