[TargetLoweringObjectFileImpl] Use llvm::transform

Fixes a build issue with libc++ configured with _LIBCPP_RAW_ITERATORS (ADL not effective)

```
llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp:1602:3: error: no matching function for call to 'transform'
  transform(HexString.begin(), HexString.end(), HexString.begin(), tolower);
  ^~~~~~~~~
```

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D80475