Improve handling of arrays of unknown bound in constant expressions.

Do not spuriously reject constexpr functions that access elements of an array
of unknown bound; this may later become valid once the bound is known. Permit
array-to-pointer decay on such arrays, but disallow pointer arithmetic (since
we do not know whether it will have defined behavior).

The standard is not clear on how this should work, but this seems to be a
decent answer.

Patch by Robert Haberlach!

llvm-svn: 301822