Fixed build files for Windows clang

Made Windows files work for Windows clang.
Verified on Linux and Windows Visual Studio
compiler to make sure nothing was broken.

Change-Id: I82815491579cdfca602660279f6831c8820249f6
Reviewed-on: https://swiftshader-review.googlesource.com/7052
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Corentin Wallez <cwallez@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
diff --git a/src/Main/BUILD.gn b/src/Main/BUILD.gn
index 361ae54..849f49e 100644
--- a/src/Main/BUILD.gn
+++ b/src/Main/BUILD.gn
@@ -14,17 +14,23 @@
 
 # Need a separate config to ensure the warnings are added to the end.
 config("swiftshader_main_private_config") {
-  if (is_clang) {
+  if (is_win) {
+    cflags = [
+      "/wd4201", # nameless struct/union
+    ]
+
+    if (is_clang) {
+      cflags += [
+        "-Wno-overloaded-virtual",
+        "-Wno-string-conversion",
+        "-Wno-sign-compare",
+      ]
+    }
+  } else {
     cflags = [
       "-DLOG_TAG=\"swiftshader_main\"",
       "-msse2",
     ]
-  } else {
-    if (host_os == "win") {
-      cflags = [
-        "/wd4201", # nameless struct/union
-      ]
-    }
   }
 }