Revert^4 "Move -Wno-enum-compare{,-switch} to external cflags"

This reverts commit 8ad44855e6.

Reason for revert: Breakage fixed, try again

Change-Id: Ia648b9513d2224a5e363b65f44f9b2f838db1d3d

cc/config/clang.go

@@ -173,19 +173,16 @@ func init() {
 		// this new warning are fixed.
 		"-Wno-null-pointer-arithmetic",
 
-		// http://b/72330874 Disable -Wenum-compare until the instances detected by this new
-		// warning are fixed.
-		"-Wno-enum-compare",
-		"-Wno-enum-compare-switch",
-
 		// Disable c++98-specific warning since Android is not concerned with C++98
 		// compatibility.
 		"-Wno-c++98-compat-extra-semi",
 	}, " "))
 
-	// Extra cflags for projects under external/ directory
+	// Extra cflags for projects under external/ directory to disable warnings that are infeasible
+	// to fix in all the external projects and their upstream repos.
 	pctx.StaticVariable("ClangExtraExternalCflags", strings.Join([]string{
-		// TODO(yikong): Move -Wno flags here
+		"-Wno-enum-compare",
+		"-Wno-enum-compare-switch",
 	}, " "))
 }