compile,link: export package name in debug_info

Add a new custom attribute to compile units containing the package name
of the package (i.e. the name after the 'package' keyword), so that
debuggers can know it when it's different from the last segment
of the package path.

Change-Id: Ieadaab6f47091aabf2f4dc42c8524452eaa6715b
Reviewed-on: https://go-review.googlesource.com/c/go/+/163677
Run-TryBot: Alessandro Arzilli <alessandro.arzilli@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

src/cmd/compile/internal/gc/main.go

@@ -497,6 +497,8 @@ func Main(archInit func(*Arch)) {
 
 	finishUniverse()
 
+	recordPackageName()
+
 	typecheckok = true
 
 	// Process top-level declarations in phases.
@@ -1406,6 +1408,18 @@ func recordFlags(flags ...string) {
 	s.P = cmd.Bytes()[1:]
 }
 
+// recordPackageName records the name of the package being
+// compiled, so that the linker can save it in the compile unit's DIE.
+func recordPackageName() {
+	s := Ctxt.Lookup(dwarf.CUInfoPrefix + "packagename." + myimportpath)
+	s.Type = objabi.SDWARFINFO
+	// Sometimes (for example when building tests) we can link
+	// together two package main archives. So allow dups.
+	s.Set(obj.AttrDuplicateOK, true)
+	Ctxt.Data = append(Ctxt.Data, s)
+	s.P = []byte(localpkg.Name)
+}
+
 // flag_lang is the language version we are compiling for, set by the -lang flag.
 var flag_lang string