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
7e5682e for other reasons.

Author: joker-eph

llvm/include/llvm/ADT/Statistic.h

@@ -131,8 +131,8 @@ class TrackingStatistic {
 
 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; }