clang: Enable in-process cc1 with flag.

With goma client 189, this should now work, and it cuts the number
of (internal) compile process executions in half, which on some systems
helps a little bit with performance.

If you don't have goma client 189 yet, this will cause every compile
job to fall back to local. It was released ~3am Easter on Feb 5 2020.

Goma should auto-update. You can see your current version in
`cat ~/goma/MANIFEST` / `type c:\src\goma\goma-win64\MANIFEST`
(the VERSION= line).

If it's out-of-date, you can explicitly update with
`~/goma/goma_ctl.py update` / `c:\src\goma\goma-win64\goma_ctl.bat update`

Bug: 1049161, b/148147812
Change-Id: I2769021721d2a767b413243905052d64917d69c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032802Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740290}

build/config/compiler/BUILD.gn

@@ -498,6 +498,12 @@ config("compiler") {
       "-Xclang",
       "-instcombine-lower-dbg-declare=0",
     ]
+
+    if (target_os != "chromeos") {
+      # TODO(https://crbug.com/1049161): Remove '-DCLANG_SPAWN_CC1=ON' from build.py instead
+      # once this change has marinated a bit.
+      cflags += [ "-fintegrated-cc1" ]
+    }
   }
 
   # C11/C++11 compiler flags setup.