c++ error recovery. Build a valid AST when trying

to recover from parse error parsing the default
argument. Patch prevents crash after spewing 100s 
of errors caused by someone who forgot to compile in c++11
mode. So no test. rdar://18508589 


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218780 91177308-0d34-0410-b5e6-96231b3b80d8