[Clang][DLCov][NFCish] Fix debugloc coverage tracking macro in Clang (#146521)
In a previous commit, the llvm-config-defined macro
LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING was renamed to
LLVM_ENABLE_DEBUGLOC_TRACKING_COVERAGE. One instance of this in Clang
remains unchanged; this patch renames it, and adds an explicit
llvm-config inclusion to ensure the define doesn't silently get removed.
NFC outside of coverage tracking builds, which we do not currently test.