Fatal warnings for C++2a / Clang 6.0

[karkhaz]

Dumping various errors that happen when trying to build cprover with Clang 6.0, which has experimental support for C++2a and gives off many new warnings.

/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/7.3.1/../../../../include/c++/7.3.1/ext/new_allocator.h:140:22: error: destructor called on non-final 'java_bytecode_parse_treet::methodt' that has virtual functions but non-virtual destructor [-Werror,-Wdelete-non-virtual-dtor]
        destroy(_Up* __p) { __p->~_Up(); }
                            ^

cbmc/src/pointer-analysis/value_set_fi.cpp:744:9: error: parentheses were disambiguated as redundant parentheses around declaration of variable named 'it' [-Werror,-Wvexing-parse]
        forall_objects(it, omt.read())
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

cbmc/src/pointer-analysis/value_set_fivr.cpp:857:9: error: parentheses were disambiguated as redundant parentheses around declaration of variable named 'it' [-Werror,-Wvexing-parse]
        forall_objects(it, omt.read())
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

cbmc/src/cpp/cpp_typecheck_resolve.cpp:591:34: error: unused variable 'next'
[-Werror,-Wunused-variable]
  resolve_identifierst::iterator next;
                                   ^