goma_link: add slash to -thinlto-prefix-replace

Prior to LLVM D77223, a slash was automatically appended to the
replacement prefix, so that -thinlto-prefix-replace=;lto.x resulted in
paths of the form "lto.x/...". Since that LLVM revision, this is no
longer true, so goma_link now needs to append it explicitly.  The new
goma_link.py works with LLVM versions before and since the change.

Bug: 1082626
Change-Id: Ia99a580c8b27eb22eabacab60c37eed1d39a3b08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2207587Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Bob Haarman <inglorion@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770676}

tools/clang/scripts/goma_link.py

@@ -452,7 +452,7 @@ class GomaLinkBase(object):
       elif not self.LTO_RE.match(param):
         final_params.append(param)
 
-    index_params.append(self.WL + self.PREFIX_REPLACE + ';' + obj_dir)
+    index_params.append(self.WL + self.PREFIX_REPLACE + ';' + obj_dir + '/')
     i = 0
     while i < len(args):
       x = args[i]