Improve error recovery in C++: when we hit 'implicit int' cases in C++,

these are usually because the parser was thoroughly confused.  In addition
to typing the value being declared as an int and hoping for the best, we
mark the value as invalid so we don't get chains of errors when it is
used downstream.  In C, implicit int actually is valid, so typing the thing
as int is good and marking it invalid is bad. :)

llvm-svn: 74266