cmd/compile/internal/types2: change inference error message

Changes the type inference error message so that the position is
proceeded by a space. cmd/go rewrites the output of gc to replace
absolute paths at the beginning of lines and those proceeded by a
space or a tab to relative paths.

Updates testdir to do the same post processing on the output
of tests as cmd/go.

Fixes #68292

Change-Id: Ie109b51143e68f6e7ab4cd19064110db0e609a7e
Reviewed-on: https://go-review.googlesource.com/c/go/+/603097
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/internal/testdir/testdir_test.go

@@ -1212,7 +1212,7 @@ func (t test) errorCheck(outStr string, wantAuto bool, fullshort ...string) (err
 	for i := range out {
 		for j := 0; j < len(fullshort); j += 2 {
 			full, short := fullshort[j], fullshort[j+1]
-			out[i] = strings.Replace(out[i], full, short, -1)
+			out[i] = replacePrefix(out[i], full, short)
 		}
 	}
 
@@ -1962,3 +1962,23 @@ func splitQuoted(s string) (r []string, err error) {
 	}
 	return args, err
 }
+
+// replacePrefix is like strings.ReplaceAll, but only replaces instances of old
+// that are preceded by ' ', '\t', or appear at the beginning of a line.
+//
+// This does the same kind of filename string replacement as cmd/go.
+// Pilfered from src/cmd/go/internal/work/shell.go .
+func replacePrefix(s, old, new string) string {
+	n := strings.Count(s, old)
+	if n == 0 {
+		return s
+	}
+
+	s = strings.ReplaceAll(s, " "+old, " "+new)
+	s = strings.ReplaceAll(s, "\n"+old, "\n"+new)
+	s = strings.ReplaceAll(s, "\n\t"+old, "\n\t"+new)
+	if strings.HasPrefix(s, old) {
+		s = new + s[len(old):]
+	}
+	return s
+}