[DWARF] NFC: Collect info used by DWARFFormValue into a helper.

[DWARF] NFC: Collect info used by DWARFFormValue into a helper.

Some forms have sizes that depend on the DWARF version, DWARF format
(32/64-bit), or the size of an address.  Collect these into a struct
to simplify passing them around.  Require callers to provide one when
they query a form's size.

Differential Revision: http://reviews.llvm.org/D34570

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306315 91177308-0d34-0410-b5e6-96231b3b80d8