Add support for hashing attributes with DW_FORM_block. This required
passing down an AsmPrinter instance so we could compute the size of
the block which could be target specific. All of the test cases in
the unittest don't have any target specific data so we can use a NULL
AsmPrinter there. This also depends upon block data being added as
integers.

We can now hash the entire fission-cu.ll compile unit so turn the
flag on there with the hash value.