[analyzer] Clean up a few doc comments for ProgramState and CallEvent.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176600 91177308-0d34-0410-b5e6-96231b3b80d8
[analyzer] Check for returning null references in ReturnUndefChecker.

Officially in the C++ standard, a null reference cannot exist. However,
it's still very easy to create one:

int &getNullRef() {
  int *p = 0;
  return *p;
}

We already check that binds to reference regions don't create null references.
This patch checks that we don't create null references by returning, either.

<rdar://problem/13364378>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176601 91177308-0d34-0410-b5e6-96231b3b80d8
Fix typo.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176603 91177308-0d34-0410-b5e6-96231b3b80d8
Add a hasExternalStorageAsWritten helper. No functionality change.

It is possible that some of the current uses of
"getStorageClassAsWritten() == SC_Extern" should use this but I don't know
enough about SC_PrivateExtern to change and test them.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176606 91177308-0d34-0410-b5e6-96231b3b80d8