Allow deprecated flags when building with LLVM_NEXT.

-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
is deprecated in the next compiler. So add -Wno-unused-command-line-argument
to skip it.

Bug: 253033919
Bug: 259559364
Test: run test_compiler.py
Change-Id: I9133a36700afd3ef411092fd8133a7fe1c2c7343
diff --git a/cc/config/global.go b/cc/config/global.go
index 9f18784..e5c0a8e 100644
--- a/cc/config/global.go
+++ b/cc/config/global.go
@@ -292,6 +292,8 @@
 	llvmNextExtraCommonGlobalCflags = []string{
 		// New warnings to be fixed after clang-r475365
 		"-Wno-error=single-bit-bitfield-constant-conversion", // http://b/243965903
+		// Skip deprecated flags.
+		"-Wno-unused-command-line-argument",
 	}
 
 	IllegalFlags = []string{