[libFuzzer][Windows]Temporarily disable /OPT:REF in fuzzer builds

/OPT:REF is currently stripping the constructors needed by sancov
clients (such as libFuzzer). Disable it for fuzzing builds until
this issue is resolved on the Clang/libFuzzer side.

Bug: 884545
Change-Id: I9c12295020e6a2efc2ade268b25a555975e71c85
Reviewed-on: https://chromium-review.googlesource.com/1249151
Commit-Queue: Jonathan Metzman <metzman@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594927}

build/config/win/BUILD.gn

@@ -136,13 +136,18 @@ config("compiler") {
     ldflags += [ "/TIMESTAMP:" + build_timestamp ]
   }
 
-  if (!is_debug && !is_component_build) {
+  if (!is_debug && !is_component_build && !use_libfuzzer) {
     # Enable standard linker optimizations like GC (/OPT:REF) and ICF in static
     # release builds. These are implied by /PROFILE below, but /PROFILE is
     # incompatible with /debug:fastlink and LLD ignores it as of this writing.
     # Release builds always want these optimizations, so enable them explicitly.
+    # TODO(crbug.com/884545): Remove the checks for use_libfuzzer when
+    # libFuzzer's issues with /OPT:REF are resolved upstream.
+    if (!use_libfuzzer) {
+      ldflags += [ "/OPT:REF" ]
+    }
+
     ldflags += [
-      "/OPT:REF",
       "/OPT:ICF",
       "/INCREMENTAL:NO",
       "/FIXED:NO",
@@ -159,7 +164,9 @@ config("compiler") {
     # PDB file by about 5%) but does not otherwise alter the output binary. It
     # is enabled opportunistically for builds where it is not prohibited (not
     # supported when incrementally linking, or using /debug:fastlink).
-    if (!is_win_fastlink) {
+    # /PROFILE implies /OPT:REF. Don't use it with libFuzzer while /OPT:REF
+    # can't be used with libFuzzer. See crbug.com/884545 for more details.
+    if (!is_win_fastlink && !use_libfuzzer) {
       ldflags += [ "/PROFILE" ]
     }
   }