Add MB configs for ClangToTLinux* bots on FYI

The tip-of-tree Clang bots rely on an environment variable
called LLVM_FORCE_HEAD_REVISION to be set in order to know
to fetch the tip-of-tree Clang and use its version in the compile
time defines, rather than the checked-in version.

We don't want GN to use or depend on environment variables,
so this patch modifies the clang update script to also look at
a new command line arg, and adds a GN arg called 'llvm_force_head_revision' to cause the same effect.

However, we need to keep the env var around for `gclient runhooks`
and to keep the existing bots (and devs) working while on GYP,
and, perhaps more importantly, adding a new GYP_DEFINE that
does what we need to build/common.gypi is proving intractable,
so we hack MB to rewrite what the GYP_DEFINE would be if life
made sense back into the env var.

BUG=481692, 582737
R=thakis@chromium.org, brettw@chromium.org

Review URL: https://codereview.chromium.org/1908293002

Cr-Original-Commit-Position: refs/heads/master@{#389408}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 909bad6afa7124f67c4890f3c1bc244179141978

config/compiler/BUILD.gn

@@ -90,6 +90,23 @@ declare_args() {
   # Gold icf level, values are "none", "safe", "all". If value isn't overridden,
   # default initialization is below.
   gold_icf_level = ""
+
+  # If this is set to true, or if LLVM_FORCE_HEAD_REVISION is set to 1
+  # in the environment, we use the revision in the llvm repo to determine
+  # the CLANG_REVISION to use, instead of the version hard-coded into
+  # //tools/clang/scripts/update.py. This should only be used in
+  # conjunction with setting LLVM_FORCE_HEAD_REVISION in the
+  # environment when `gclient runhooks` is run as well.
+  llvm_force_head_revision = false
+}
+
+if (is_clang && !is_nacl) {
+  update_args = [ "--print-revision" ]
+  if (llvm_force_head_revision) {
+    update_args += [ "--llvm-force-head-revision" ]
+  }
+  clang_revision =
+      exec_script("//tools/clang/scripts/update.py", update_args, "trim string")
 }
 
 # Apply the default logic for these values if they were not set explicitly.
@@ -226,10 +243,7 @@ config("compiler") {
     # when turning clang on or off. (defines are passed via the command line,
     # and build system rebuild things when their commandline changes). Nothing
     # should ever read this define.
-    defines +=
-        [ "CR_CLANG_REVISION=" + exec_script("//tools/clang/scripts/update.py",
-                                             [ "--print-revision" ],
-                                             "trim string") ]
+    defines += [ "CR_CLANG_REVISION=$clang_revision" ]
   }
 
   # Non-Mac Posix compiler flags setup.
@@ -974,9 +988,7 @@ config("default_warnings") {
         "-Wno-shift-negative-value",
       ]
 
-      if (exec_script("//tools/clang/scripts/update.py",
-                      [ "--print-revision" ],
-                      "trim string") != "266460-1") {
+      if (llvm_force_head_revision) {
         cflags += [
           # TODO(thakis): https://crbug.com/604888
           "-Wno-undefined-var-template",