Stop rebuilding the world whenever a diagnostic changes

The inclusion of the full diagnostic list (via DiagnosticList.h) in
DiagnosticGroups.h meant that touching any diagnostic caused most of
the world to rebuild, making me sad. Use forward declarations to limit
how much needs to be rebuilt when changing diagnostics.

Author: DougGregor

include/swift/AST/DiagnosticGroups.h

@@ -18,13 +18,13 @@
 #ifndef SWIFT_DIAGNOSTICGROUPS_H
 #define SWIFT_DIAGNOSTICGROUPS_H
 
-#include "swift/AST/DiagnosticList.h"
 #include "llvm/ADT/ArrayRef.h"
 #include <array>
 #include <string_view>
 #include <unordered_map>
 
 namespace swift {
+enum class DiagID : uint32_t;
 
 enum class DiagGroupID : uint16_t {
 #define GROUP(Name, Version) Name,

lib/AST/DiagnosticGroups.cpp

@@ -16,6 +16,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "swift/AST/DiagnosticGroups.h"
+#include "swift/AST/DiagnosticList.h"
 #include <unordered_set>
 
 namespace swift {