DWARFFormValue: Add getAsSignedConstant method.

The implementation accepts explicitely signed forms (DW_FORM_sdata),
but also unsigned forms as long as they fit in an int64_t.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231299 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed