[Wconversion] Suppress warnings

Bug: 56258
Bug: 58162
Change-Id: Ib0a6fcc420d61c56dc1b32260d91a9f6a40440ce
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/spirv-tools/+/422717
Reviewed-by: Petr Hosek <phosek@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 52487de..bea2a4d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -23,6 +23,7 @@
   visibility = [ ":*" ]
   cflags_cc = [
     "-Wno-delete-non-virtual-dtor",
+    "-Wno-conversion",
   ]
 }