clang: Disable -instcombine-lower-dbg-declare=1 for CrOS builds, take 2

The previous attempt (#502668) didn't actually work.

TBR=llozano

Bug: 766311,765793
Change-Id: I3ea9c11b9c2f320e2989723f1d22f4eac75e6583
Reviewed-on: https://chromium-review.googlesource.com/671471
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#502685}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e1cd7e67e4f7039c1922ccf11521e36fdbcb85fd

config/compiler/BUILD.gn

@@ -443,7 +443,7 @@ config("compiler") {
   # TODO(hans): Remove this once Clang generates better optimized debug info by
   # default. https://crbug.com/765793
   if (is_clang && !is_nacl && current_toolchain == host_toolchain &&
-      !is_chromeos) {
+      target_os != "chromeos") {
     cflags += [
       "-Xclang",
       "-mllvm",