clang: disable -Wunreachable-code-loop-increment

There're four instances of this in clang. One of them appears to be a
legit bug, but it's in module bitcode deserialization code. I assume we
never use that, and the bug causes clang to emit errors super eagerly,
so I'm leaving it untouched, since `git blame` says the code is >5yrs
old.

If anyone's interested, the bug is at

external/clang/lib/Serialization/ASTReader.cpp:3304

Bug: 150166387
Test: TreeHugger
Change-Id: I45a1abff8f8abd1ed77d752be252ebf65a69b898
diff --git a/Android.bp b/Android.bp
index 71d7cd1..8be9001 100644
--- a/Android.bp
+++ b/Android.bp
@@ -10,8 +10,9 @@
         "-pedantic",
         "-Wno-cast-qual",
         "-Wno-long-long",
-        "-Wno-unused-private-field",
+        "-Wno-unreachable-code-loop-increment",
         "-Wno-unused-lambda-capture",
+        "-Wno-unused-private-field",
     ],
 
     include_dirs: [