goma_link: parse `,` from `-mllvm` args appropriately

CrOS builders are complaining about broken flags like:
`-mllvm ,-enable-dse-memoryssa=false`

We should probably be dropping that `,`. This CL makes us do that.

Bug: 1135234
Change-Id: I3f99498003282d8e737d3d066bc7e2e257f65dec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2450743Reviewed-by: Bob Haarman <inglorion@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813878}

tools/clang/scripts/goma_link.py

@@ -237,7 +237,7 @@ class GomaLinkBase(object):
       '-Wl,--lto.*',
       '-Wl,--thin.*',
   )))
-  MLLVM_RE = re.compile('(?:-Wl,)?([-/]mllvm)[:=]?(.*)', re.IGNORECASE)
+  MLLVM_RE = re.compile('(?:-Wl,)?([-/]mllvm)[:=,]?(.*)', re.IGNORECASE)
   OBJ_RE = re.compile('(.*)\\.(o(?:bj)?)', re.IGNORECASE)
 
   def _no_codegen(self, args):

tools/clang/scripts/goma_link_unit_tests.py

@@ -228,6 +228,13 @@ class GomaLinkUnitTest(unittest.TestCase):
       self.assertTrue(os.path.exists('test'))
       self.assertRaises(OSError, goma_link.ensure_file, 'test/impossible')
 
+  def test_transform_codegen_param_on_mllvm(self):
+    # Regression test for crbug.com/1135234
+    link = goma_ld.GomaLinkUnix()
+    self.assertEqual(
+        link.transform_codegen_param_common('-mllvm,-import-instr-limit=20'),
+        ['-mllvm', '-import-instr-limit=20'])
+
 
 if __name__ == '__main__':
   unittest.main()