[clang] [Clang][DLCov][NFCish] Fix debugloc coverage tracking macro in Clang (PR #146521)

[Stephen Tozer via cfe-commits]

SLTozer wrote:

> Can you explain how the include helps in a bit more detail? I don't see what it adds (I'm not very familiar with llvm-config!)

It changes nothing now, but llvm-config contains the macro definition used here - adding the include explicitly ensures that we don't silently lose the macro definition if all the headers that were including llvm-config are removed (or have their llvm-config inclusions removed).

https://github.com/llvm/llvm-project/pull/146521