[NFC][Lanai][Hexagon] Define DEBUG_TYPE after including Debug.h. (#125440)
This patch fixes some warnings/errors that occur with certain
compilers-- due to existing code not following the idiom mentioned in
`Debug.h`:
```
// LLVM_DEBUG() requires the DEBUG_TYPE macro to be defined. Set it to "foo"
// specify that your debug code belongs to class "foo". **Be careful that you only
// do this after including Debug.h and not around any #include of headers.**
```