[DebugInfo][DWARF] Add doesFormBelongToClass function.

The result of DWARFFormValue::isFormClass depends on DWARF version in some cases.
The current implementation takes DWARF version from the stored DWARFUnit.
If there is no stored DWARFUnit then the current behavior is to assume
DwarfVersion <= 3. This patch adds new function which has a DWARF version as a
parameter so it is possible to check form class for various DWARF versions.

Differential Revision: https://reviews.llvm.org/D145499