cmd/compile: fix crash in importer when running in debug mode

Verified by manually enabling debug mode and running make.bash.

Fixes #20684.

Change-Id: I041f2ca6ef1d4198815724d98f61511072d63581
Reviewed-on: https://go-review.googlesource.com/45971
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -564,7 +564,12 @@ func (p *exporter) pos(n *Node) {
 
 func (p *exporter) path(s string) {
 	if i, ok := p.pathIndex[s]; ok {
-		p.index('p', i) // i >= 0
+		// Note: Using p.index(i) here requires the use of p.tag(-len(c)) below
+		//       to get matching debug markers ('t'). But in trace mode p.tag
+		//       assumes that the tag argument is a valid tag that can be looked
+		//       up in the tagString list, rather then some arbitrary slice length.
+		//       Use p.int instead.
+		p.int(i) // i >= 0
 		return
 	}
 	p.pathIndex[s] = len(p.pathIndex)

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

@@ -420,12 +420,9 @@ func (p *importer) pos() src.XPos {
 }
 
 func (p *importer) path() string {
-	if p.debugFormat {
-		p.marker('p')
-	}
 	// if the path was seen before, i is its index (>= 0)
 	// (the empty string is at index 0)
-	i := p.rawInt64()
+	i := p.int()
 	if i >= 0 {
 		return p.pathList[i]
 	}