go/analysis/passes/fieldalignment: fix data race

This CL is a quick band-aid over a data race spotted in passing.
A principled solution can wait for another day.
Fortunately fieldalignment isn't much used now.

Updates golang/go#43233

Change-Id: I5643294e3429c5c0b152aab046c2f78065c755be
Reviewed-on: https://go-review.googlesource.com/c/tools/+/716603
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

Author: adonovan

go/analysis/passes/fieldalignment/fieldalignment.go

@@ -18,6 +18,7 @@ import (
 	"golang.org/x/tools/go/analysis"
 	"golang.org/x/tools/go/analysis/passes/inspect"
 	"golang.org/x/tools/go/ast/inspector"
+	"golang.org/x/tools/internal/astutil"
 )
 
 const Doc = `find structs that would use less memory if their fields were sorted
@@ -103,6 +104,11 @@ func fieldalignment(pass *analysis.Pass, node *ast.StructType, typ *types.Struct
 		return
 	}
 
+	// Analyzers borrow syntax tree; they do not own them and must modify them.
+	// This Clone operation is a quick fix to the data race introduced
+	// in CL 278872 by the clearing of the Comment and Doc fields below.
+	node = astutil.CloneNode(node)
+
 	// Flatten the ast node since it could have multiple field names per list item while
 	// *types.Struct only have one item per field.
 	// TODO: Preserve multi-named fields instead of flattening.