AstGen: remove unneeded field ref_start_index

Previously, this field was used because the Zir.Inst.Ref encoding
supported the concept of references to function parameters. However now
thanks to whole-file-astgen, the implementations of indexToRef and
refToIndex are trivial addition/subtraction of a comptime const integer.
9918a5fbe3Andrew Kelley committed on 7/20/2021, 7:19:14 PMparent3c3abaf
1 file changedLine totals unavailable