Build with _LIBCPP_ENABLE_NODISCARD

Also rolls buildtools [1] and includes some small cleanups.

[1] https://chromium.googlesource.com/chromium/buildtools/+log/2dff9c9c74e9d732e6fe57c84ef7fd044cc45d96..13a00f110ef910a25763346d6538b60f12845656

BUG=894427
R=thakis

Change-Id: I52fd273ab2e93f8c5e461fd3bf7a34b09135d384
Reviewed-on: https://chromium-review.googlesource.com/c/1277374
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599734}

DEPS

@@ -121,7 +121,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling build tools
   # and whatever else without interference from each other.
-  'buildtools_revision': '2dff9c9c74e9d732e6fe57c84ef7fd044cc45d96',
+  'buildtools_revision': '13a00f110ef910a25763346d6538b60f12845656',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling SwiftShader
   # and whatever else without interference from each other.

build/config/posix/BUILD.gn

@@ -42,12 +42,6 @@ config("runtime_library") {
         defines += [ "_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS" ]
       }
     }
-    if (!is_clang) {
-      # Gcc has a built-in abs() definition with default visibility.
-      # If it was not disabled, it would conflict with libc++'s abs()
-      # with hidden visibility.
-      cflags += [ "-fno-builtin-abs" ]
-    }
     cflags_cc += [
       "-nostdinc++",
       "-isystem" + rebase_path("$libcxx_prefix/include", root_build_dir),
@@ -56,6 +50,7 @@ config("runtime_library") {
     defines += [
       "CR_LIBCXX_REVISION=$libcxx_svn_revision",
       "CR_LIBCXXABI_REVISION=$libcxxabi_svn_revision",
+      "_LIBCPP_ENABLE_NODISCARD",
     ]
 
     # Make sure we don't link against libc++ or libstdc++.
@@ -67,6 +62,11 @@ config("runtime_library") {
         ldflags += [ "-nostdlib++" ]
       }
     } else {
+      # Gcc has a built-in abs() definition with default visibility.
+      # If it was not disabled, it would conflict with libc++'s abs()
+      # with hidden visibility.
+      cflags += [ "-fno-builtin-abs" ]
+
       ldflags += [ "-nodefaultlibs" ]
 
       # Unfortunately, there's no way to disable linking against just libc++

build/config/win/BUILD.gn

@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//buildtools/deps_revisions.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/c++/c++.gni")
 import("//build/config/clang/clang.gni")
@@ -256,6 +257,8 @@ config("runtime_library") {
     defines += [ "_USING_V110_SDK71_" ]
   }
 
+  # TODO(thomasanderson): Move this into a target in //build/config/c++ and
+  # deduplicate with //build/config/posix/BUILD.gn.
   if (use_custom_libcxx) {
     cflags_cc +=
         [ "-I" + rebase_path("$libcxx_prefix/include", root_build_dir) ]
@@ -263,11 +266,17 @@ config("runtime_library") {
       defines += [ "_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS" ]
     }
 
-    # Prevent libc++ from embedding linker flags to try to automatically link
-    # against its runtime library. This is unnecessary with our build system,
-    # and can also result in build failures if libc++'s name for a library does
-    # not match ours.
-    defines += [ "_LIBCPP_NO_AUTO_LINK" ]
+    defines += [
+      "CR_LIBCXX_REVISION=$libcxx_svn_revision",
+      "CR_LIBCXXABI_REVISION=$libcxxabi_svn_revision",
+      "_LIBCPP_ENABLE_NODISCARD",
+
+      # Prevent libc++ from embedding linker flags to try to automatically link
+      # against its runtime library. This is unnecessary with our build system,
+      # and can also result in build failures if libc++'s name for a library
+      # does not match ours.
+      "_LIBCPP_NO_AUTO_LINK",
+    ]
   }
 
   if (current_os == "winuwp") {