cgo: add file AST for fake C file locations

This is needed for the type checker, otherwise it doesn't know which Go
version it should use for type checking.

cgo/cgo_go122.go

@@ -0,0 +1,17 @@
+//go:build go1.22
+
+package cgo
+
+// Code specifically for Go 1.22.
+
+import (
+	"go/ast"
+	"go/token"
+)
+
+func init() {
+	setASTFileFields = func(f *ast.File, start, end token.Pos) {
+		f.FileStart = start
+		f.FileEnd = end
+	}
+}