Disallow explicit cast to nt_array_ptr in checked scopes (#391)

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#626

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Disallow cast from other checked pointer types to nt_array_ptr in
checked scopes because the source pointer might not point to a
NULL_terminated array. Casting from an unchecked pointer to a
nt_array_ptr pointer should also be prohibited; this has already been
handled as no unchecked pointers are allowed in checked scopes.

Also added a new error message in
clang/include/clang/Basic/DiagnosticSemaKinds.td for casting to
nt_array_ptr in checked scopes.

The test file tests/typechecking/checked_scope_basic.c was updated
with a new function test_cast_to_nt_array_ptr to test
casting to nt_array_ptr.

The change passed the new test code and the regression tests for
checkedc and clang.