Disable -Wno-conversion on MSVC compiler (#2410)

diff --git a/BUILD.gn b/BUILD.gn
index f877078..29256c4 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -76,7 +76,9 @@
 
 config("glslang_public") {
   include_dirs = [ "." ]
-  cflags = [ "-Wno-conversion" ]
+  if (!is_win || is_clang) {
+    cflags = [ "-Wno-conversion" ]
+  }
 }
 
 config("glslang_hlsl") {