Disable /guard:cf,nolongjmp in ThinLTO configurations where it breaks.

crbug.com/818086 shows that using control flow guard, ThinLTO, and
RTTI together results in broken binaries. This change disables CFG
when ThinLTO and RTTI are enabled to allow builds to succeed while a
fix for the problem is developed.

R=dpranke,thakis

Bug: 818086
Change-Id: Ia926350dc4193f3a1c87a45b36a8d5dc1dd97c08
Reviewed-on: https://chromium-review.googlesource.com/952666Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541646}

build/config/win/BUILD.gn

@@ -302,7 +302,13 @@ config("cfi_linker") {
   # turned on either.
   # CFG seems to lead to random corruption with incremental linking so turn off
   # CFG in component builds. https://crbug.com/812421
-  if (!is_debug && !is_component_build) {
+  # CFG, ThinLTO and RTTI don't currently work together, so turn off
+  # CFG for that combination. We detect RTTI by looking for
+  # use_cfi_diag || is_ubsan_vptr || is_ubsan_security, as per
+  # //build/config/compiler/BUILD.gn.
+  # TODO(inglorion): Re-enable once figured out, bug https://crbug.com/818086
+  if (!is_debug && !is_component_build &&
+      !(use_thin_lto && (use_cfi_diag || is_ubsan_vptr || is_ubsan_security))) {
     # Turn on CFG in msvc linker, regardless of compiler used. Turn off CFG for
     # longjmp (new in VS 2017) because it relies on compiler support which we do
     # not have enabled.