Teach static analyzer about AttributedStmts

Summary:
Static analyzer used to abort when encountering AttributedStmts, because it
asserted that the statements should not appear in the CFG. This is however not
the case, since at least the clang::fallthrough annotation makes it through.

This commit simply makes the analyzer ignore the statement attributes.

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1030