clang tot bots: Force-enable memoryssa-based dse

Now that the flag is off-by-default again upstream, let's force it on
on the tot bots to see if things work with it on.

TBR=akhuang

Bug: 1127713
Change-Id: Ib9267b5123472b1c28280d858da7229b140a3b7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415222Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807727}

build/config/compiler/BUILD.gn

@@ -505,7 +505,7 @@ config("compiler") {
       cflags += [
         # TODO(https://crbug.com/1127713): Investigate, remove.
         "-mllvm",
-        "-enable-dse-memoryssa=false",
+        "-enable-dse-memoryssa=true",
       ]
     }
   }