Fix to PR8880 (clang dies processing a for loop).

Due to statement expressions supported as GCC extension, it is possible
to put 'break' or 'continue' into a loop/switch statement but outside its
body, for example:

    for ( ; ({ if (first) { first = 0; continue; } 0; }); )

Such usage must be diagnosed as an error, GCC rejects it. To recognize
this and similar patterns the flags BreakScope and ContinueScope are
temporarily turned off while parsing condition expression.

Differential Revision: http://llvm-reviews.chandlerc.com/D1762

llvm-svn: 193073