Make NoopStatistic constructor constexpr (NFC) (#159045)

This makes it friendly to -Wglobal-constructors environments. This class
is used when Statistics are disabled, the matching class,
TrackingStatistic, was made constexpr a while back already in
7e5682ee6201c for other reasons.
diff --git a/llvm/include/llvm/ADT/Statistic.h b/llvm/include/llvm/ADT/Statistic.h
index 082e6d5..795b0c2 100644
--- a/llvm/include/llvm/ADT/Statistic.h
+++ b/llvm/include/llvm/ADT/Statistic.h
@@ -131,8 +131,8 @@
 
 class NoopStatistic {
 public:
-  NoopStatistic(const char * /*DebugType*/, const char * /*Name*/,
-                const char * /*Desc*/) {}
+  constexpr NoopStatistic(const char * /*DebugType*/, const char * /*Name*/,
+                          const char * /*Desc*/) {}
 
   uint64_t getValue() const { return 0; }