Introduce revng_log and abandon DBG(...)

This commit introduces `revng_log`, a macro analagous to `revng_assert`,
which basically allows to have the benefit of the `Logger` class without
having to compute the expression to log if the logger is disabled.

This commit also completely dismisses the `DBG` macro, converting all
the old code to `Logger` + `revng_log`.

lib/BasicAnalyses/FunctionCallIdentification.cpp

@@ -20,7 +20,7 @@ static RegisterPass<FunctionCallIdentification> X("fci",
                                                   true);
 
 bool FunctionCallIdentification::runOnFunction(llvm::Function &F) {
-  DBG("passes", { dbg << "Starting FunctionCallIdentification\n"; });
+  revng_log(PassesLog, "Starting FunctionCallIdentification");
 
   auto &GCBI = getAnalysis<GeneratedCodeBasicInfo>();
 
@@ -230,7 +230,7 @@ bool FunctionCallIdentification::runOnFunction(llvm::Function &F) {
     }
   }
 
-  DBG("passes", { dbg << "Ending FunctionCallIdentification\n"; });
+  revng_log(PassesLog, "Ending FunctionCallIdentification");
 
   return false;
 }