Turn on string-plus-int warning

All instances for the warning are fixed/suppressed, turn the warning
back on.

Test: m checkbuild
Bug: 128878287
Change-Id: Ifd5f9a64cb7ff1ce47f498ed38436056b493ee92
diff --git a/cc/config/clang.go b/cc/config/clang.go
index 22969eb..81439f3 100644
--- a/cc/config/clang.go
+++ b/cc/config/clang.go
@@ -178,10 +178,6 @@
 
 		// Disable this warning because we don't care about behavior with older compilers.
 		"-Wno-return-std-move-in-c++11",
-
-		// Disable -Wstring-plus-int until the instances detected by this new warning is
-		// fixed.
-		"-Wno-string-plus-int",
 	}, " "))
 
 	// Extra cflags for projects under external/ directory