Change "enum element" to "enum case" in diagnostics

From the perspective of the compiler implementation, they're elements. But users will think of these as cases—and many diagnostics already refer to these as enum cases.

lib/AST/Decl.cpp

@@ -279,7 +279,7 @@ StringRef Decl::getDescriptiveKindName(DescriptiveDeclKind K) {
   ENTRY(MaterializeForSet, "materializeForSet accessor");
   ENTRY(Addressor, "address accessor");
   ENTRY(MutableAddressor, "mutableAddress accessor");
-  ENTRY(EnumElement, "enum element");
+  ENTRY(EnumElement, "enum case");
   ENTRY(Module, "module");
   ENTRY(MissingMember, "missing member placeholder");
   ENTRY(Requirement, "requirement");