cmd/compile: workaround for inconsistent receiver param srcpos

Given an inlinable method M in package P:

   func (r *MyStruct) M(...) {

When M is compiled within its home package, the source position that
the compiler records for 'r' (receiver parameter variable) is
accurate, whereas if M is built as part of the compilation of some
other package (body read from export data), the declaration line
assigned to 'r' will be the line number of the 'import' directive, not
the source line from M's source file.

This inconsistency can cause differences in the size of abstract
parameter DIEs (due to variable-length encoding), which can then in
turn result in bad abstract origin offsets, which in turn triggers
build failures on iOS (dsymutil crashes when it encounters an
incorrect abstract origin reference).

Work around the problem by removing the "declaration line number"
attribute within the abstract parameter abbreviation table entry. The
decl line attribute doesn't contribute a whole lot to the debugging
experience, and it gets rid of the inconsistencies that trigger the
dsymutil crashes.

Updates #23374.

Change-Id: I0fdc8e19a48db0ccd938ceadf85103936f89ce9f
Reviewed-on: https://go-review.googlesource.com/87055
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

src/cmd/internal/dwarf/dwarf.go

@@ -551,7 +551,6 @@ var abbrevs = [DW_NABRV]dwAbbrev{
 		[]dwAttrForm{
 			{DW_AT_name, DW_FORM_string},
 			{DW_AT_variable_parameter, DW_FORM_flag},
-			{DW_AT_decl_line, DW_FORM_udata},
 			{DW_AT_type, DW_FORM_ref_addr},
 		},
 	},
@@ -1429,7 +1428,10 @@ func putAbstractVar(ctxt Context, info Sym, v *Var) {
 	}
 
 	// Line
-	putattr(ctxt, info, abbrev, DW_FORM_udata, DW_CLS_CONSTANT, int64(v.DeclLine), nil)
+	if abbrev != DW_ABRV_PARAM_ABSTRACT {
+		// See issue 23374 for more on why decl line is skipped for abs params.
+		putattr(ctxt, info, abbrev, DW_FORM_udata, DW_CLS_CONSTANT, int64(v.DeclLine), nil)
+	}
 
 	// Type
 	putattr(ctxt, info, abbrev, DW_FORM_ref_addr, DW_CLS_REFERENCE, 0, v.Type)