Fixed two problems when reading constant/register
variables in the ValueObject code:

  - Report an error if the variable does not have
    a valid address.

  - Return the contents of the data to GetData(),
    even if the value is constant.

<rdar://problem/13690855>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@179876 91177308-0d34-0410-b5e6-96231b3b80d8