Mark STATISTIC variables as maybe_unused when stats are disabled. (llvm#159103)

PR llvm#159045 made the constructor constexpr, which allows
`-Wunused-variable` to trigger. However, we don't really care if a
statistic is unused if `LLVM_ENABLE_STATS` is 0.

Author: jyknight

llvm/include/llvm/ADT/Statistic.h

@@ -164,8 +164,13 @@ using Statistic = NoopStatistic;
 
 // STATISTIC - A macro to make definition of statistics really simple.  This
 // automatically passes the DEBUG_TYPE of the file into the statistic.
+#if LLVM_ENABLE_STATS
 #define STATISTIC(VARNAME, DESC)                                               \
   static llvm::Statistic VARNAME = {DEBUG_TYPE, #VARNAME, DESC}
+#else
+#define STATISTIC(VARNAME, DESC)                                               \
+  static llvm::Statistic VARNAME [[maybe_unused]] = {DEBUG_TYPE, #VARNAME, DESC}
+#endif
 
 // ALWAYS_ENABLED_STATISTIC - A macro to define a statistic like STATISTIC but
 // it is enabled even if LLVM_ENABLE_STATS is off.