[llvm] [ADT] Adding bidirectional iterator functionality + unit tests (PR #160726)

[Jakub Kuderski via llvm-commits]

================
@@ -1177,6 +1178,98 @@ TYPED_TEST(BitVectorTest, Iterators) {
     EXPECT_EQ(List[i++], Bit);
 }
 
+TYPED_TEST(BitVectorTest, BidirectionalIterator) {
+  // Test decrement operators
----------------
kuhar wrote:

Nit:  
```suggestion
  // Test decrement operators.
```
See https://llvm.org/docs/CodingStandards.html#commenting.

The same applies to the other comments below.

https://github.com/llvm/llvm-project/pull/160726