Disable new clang warnings in llvm-10.0

This is old code, so there's not much point in flagging warnings.

Bug: chromium:403236787, chromium:368047728
Change-Id: Ib3bbd15036258b1ecef7a04dbe7254196f0caf73
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/75288
Tested-by: Shahbaz Youssefi <syoussefi@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@google.com>

third_party/llvm-10.0/BUILD.gn

diff --git a/third_party/llvm-10.0/BUILD.gn b/third_party/llvm-10.0/BUILD.gn
index 59e5230..1c3dafa 100644
--- a/third_party/llvm-10.0/BUILD.gn
+++ b/third_party/llvm-10.0/BUILD.gn
@@ -72,6 +72,8 @@
       "-Wno-unused-private-field",
       "-Wno-unused-result",
       "-Wno-unused-variable",
+      "-Wno-unnecessary-virtual-specifier",
+      "-Wno-unique-object-duplication",
     ]
   }