Expand ClangExtraExternalCflags to non-Google vendor projects

Some of the warnings are too common to fix/opt-out for non-Google
projects.

Also in the change, minor clean up of duplicated code.

Test: presubmit
Bug: 139945549
Change-Id: Ic176ef1f17133405851a79592b6bef5ccb403bd9
diff --git a/cc/config/clang.go b/cc/config/clang.go
index 030a076..eddc341 100644
--- a/cc/config/clang.go
+++ b/cc/config/clang.go
@@ -163,8 +163,8 @@
 		"-Wno-tautological-type-limit-compare",
 	}, " "))
 
-	// Extra cflags for projects under external/ directory to disable warnings that are infeasible
-	// to fix in all the external projects and their upstream repos.
+	// Extra cflags for external third-party projects to disable warnings that
+	// are infeasible to fix in all the external projects and their upstream repos.
 	pctx.StaticVariable("ClangExtraExternalCflags", strings.Join([]string{
 		"-Wno-enum-compare",
 		"-Wno-enum-compare-switch",