Reland "clang tot bots: Force-enable memoryssa-based dse"

This is a reland of a359d11d
This time, leave the force-off on non-tot bots in place unchanged,
and add a block to force-on on tot bots, and add a somewhat detailed
comment for why. Third time's the charm?

Original change's description:
> 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/+/2415222
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#807727}

TBR=akhuang

Bug: 1127713
Change-Id: I154cb9f12c2500e715fe9215bd87e96cc03c5040
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2416768Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807900}

build/config/compiler/BUILD.gn

@@ -501,6 +501,10 @@ config("compiler") {
       "-instcombine-lower-dbg-declare=0",
     ]
 
+    # Pinned clang has enable-dse-memoryssa enabled by default but it's broken,
+    # so we have to force it off.
+    # Trunk clang has it disabled by default but it might work, so we force it on
+    # so the ToT bots can check if it works now.
     if (!llvm_force_head_revision) {
       cflags += [
         # TODO(https://crbug.com/1127713): Investigate, remove.
@@ -508,6 +512,13 @@ config("compiler") {
         "-enable-dse-memoryssa=false",
       ]
     }
+    if (llvm_force_head_revision) {
+      cflags += [
+        # TODO(https://crbug.com/1127713): Investigate, remove.
+        "-mllvm",
+        "-enable-dse-memoryssa=true",
+      ]
+    }
   }
 
   # C11/C++11 compiler flags setup.