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
1 file changed