lldb: remove adhoc implementation of array_sizeof

Replace adhoc inline implementation of llvm::array_lengthof in favour of the
implementation in LLVM.  This is simply a cleanup change, no functional change
intended.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@211868 91177308-0d34-0410-b5e6-96231b3b80d8