[libclang] CoroutineBody/Coreturn statements are UnexposedStmts and not Exprs

This change ensures that the libclang CXCursor represents the CoroutineBody
and the Coreturn statement using the appropriate CXCursor_UnexposedStmt kind
instead of CXCursor_UnexposedExpr. The problem with CXCursor_UnexposedExpr is
that the consumer functions assumed that CoroutineBody/Coreturn statements
were valid expressions and performed an invalid downcast to Expr causing
assertion failures or other crashes.

rdar://40204290

llvm-svn: 350282