[Wconversion] Suppress glslang issue

The glslang public includes dir contains headers with implicit
conversion issues. Add -Wno-conversion to glslang's public config.

Bug: 60140
Bug: 58162
Change-Id: Iec27cb4242e9fdceddd6a3e02044a0bccfa0ce36
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/glslang/+/429054
Reviewed-by: Petr Hosek <phosek@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 80e977c..d8e14ae 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -52,6 +52,8 @@
   include_dirs = [ "." ]
 
   defines = [ "ENABLE_HLSL=1" ]
+
+  cflags = [ "-Wno-conversion" ]
 }
 
 source_set("glslang_sources") {
@@ -183,7 +185,6 @@
       "-Wno-sign-compare",
       "-Wno-unused-variable",
       "-Wno-unknown-warning-option",
-      "-Wno-conversion",
       "-Wno-missing-field-initializers",
       "-Wno-newline-eof",
     ]