The C++ spec (3.6.1.3) says "The function `main` shall not be used within a program". This implies that it cannot recurse, so add the norecurse attribute to help the midend out a bit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@252902 91177308-0d34-0410-b5e6-96231b3b80d8