[ADT] Use "static inline" to initialize EmptyDivergenceDesc; (NFC) (#160445)

In C++17, we can initialize a static member variable with "static
inline" as part of the class definition.  With this, we can eliminate
the out-of-line static initializer involving boilerplate template
code.

Author: kazutakahirata

llvm/include/llvm/ADT/GenericUniformityImpl.h

@@ -310,7 +310,7 @@ template <typename ContextT> class GenericSyncDependenceAnalysis {
   const DivergenceDescriptor &getJoinBlocks(const BlockT *DivTermBlock);
 
 private:
-  static DivergenceDescriptor EmptyDivergenceDesc;
+  static inline DivergenceDescriptor EmptyDivergenceDesc;
 
   ModifiedPO CyclePO;
 
@@ -741,10 +741,6 @@ template <typename ContextT> class DivergencePropagator {
   }
 };
 
-template <typename ContextT>
-typename llvm::GenericSyncDependenceAnalysis<ContextT>::DivergenceDescriptor
-    llvm::GenericSyncDependenceAnalysis<ContextT>::EmptyDivergenceDesc;
-
 template <typename ContextT>
 llvm::GenericSyncDependenceAnalysis<ContextT>::GenericSyncDependenceAnalysis(
     const ContextT &Context, const DominatorTreeT &DT, const CycleInfoT &CI)