Avoid #include AST headers in Basic headers

Resolve a layering violation. `DiagnosticBehavior` was used in
`LangOptions`. Introduce a dedicated 'enum' for the possible values.

include/swift/Basic/CXXStdlibKind.h

@@ -13,6 +13,10 @@
 #ifndef SWIFT_BASIC_CXX_STDLIB_KIND_H
 #define SWIFT_BASIC_CXX_STDLIB_KIND_H
 
+#include "llvm/Support/ErrorHandling.h"
+#include <stdint.h>
+#include <string>
+
 namespace swift {
 
 enum class CXXStdlibKind : uint8_t {

include/swift/Basic/LangOptions.h

@@ -18,7 +18,6 @@
 #ifndef SWIFT_BASIC_LANGOPTIONS_H
 #define SWIFT_BASIC_LANGOPTIONS_H
 
-#include "swift/AST/DiagnosticsFrontend.h"
 #include "swift/Basic/CXXStdlibKind.h"
 #include "swift/Basic/Feature.h"
 #include "swift/Basic/FixedBitSet.h"
@@ -46,6 +45,7 @@
 namespace swift {
 
   struct DiagnosticBehavior;
+  class DiagnosticEngine;
 
   /// Kind of implicit platform conditions.
   enum class PlatformConditionKind {
@@ -217,8 +217,14 @@ namespace swift {
 
     /// Diagnostic level to report when a public declarations doesn't declare
     /// an introduction OS version.
-    std::optional<DiagnosticBehavior> RequireExplicitAvailability =
-        std::nullopt;
+    enum class RequireExplicitAvailabilityDiagnosticBehavior : uint8_t {
+      Ignore,
+      Warning,
+      Error,
+    };
+    RequireExplicitAvailabilityDiagnosticBehavior
+        RequireExplicitAvailabilityBehavior =
+            RequireExplicitAvailabilityDiagnosticBehavior::Ignore;
 
     /// Introduction platform and version to suggest as fix-it
     /// when using RequireExplicitAvailability.

lib/AST/DiagnosticEngine.cpp

@@ -16,6 +16,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "swift/AST/DiagnosticEngine.h"
+#include "swift/AST/DiagnosticsFrontend.h"
 #include "swift/AST/ASTContext.h"
 #include "swift/AST/ASTPrinter.h"
 #include "swift/AST/Decl.h"

lib/Frontend/CompilerInvocation.cpp

@@ -1167,11 +1167,14 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   if (const Arg *A = Args.getLastArg(OPT_require_explicit_availability_EQ)) {
     StringRef diagLevel = A->getValue();
     if (diagLevel == "warn") {
-      Opts.RequireExplicitAvailability = DiagnosticBehavior::Warning;
+      Opts.RequireExplicitAvailabilityBehavior =
+          LangOptions::RequireExplicitAvailabilityDiagnosticBehavior::Warning;
     } else if (diagLevel == "error") {
-      Opts.RequireExplicitAvailability = DiagnosticBehavior::Error;
+      Opts.RequireExplicitAvailabilityBehavior =
+          LangOptions::RequireExplicitAvailabilityDiagnosticBehavior::Error;
     } else if (diagLevel == "ignore") {
-      Opts.RequireExplicitAvailability = std::nullopt;
+      Opts.RequireExplicitAvailabilityBehavior =
+          LangOptions::RequireExplicitAvailabilityDiagnosticBehavior::Ignore;
     } else {
       Diags.diagnose(SourceLoc(),
                      diag::error_unknown_require_explicit_availability,
@@ -1180,7 +1183,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   } else if (Args.getLastArg(OPT_require_explicit_availability,
                              OPT_require_explicit_availability_target) ||
              Opts.LibraryLevel == LibraryLevel::API) {
-    Opts.RequireExplicitAvailability = DiagnosticBehavior::Warning;
+    Opts.RequireExplicitAvailabilityBehavior =
+        LangOptions::RequireExplicitAvailabilityDiagnosticBehavior::Warning;
   }
 
   if (const Arg *A = Args.getLastArg(OPT_require_explicit_availability_target)) {

lib/Sema/TypeCheckAvailability.cpp

@@ -4651,10 +4651,21 @@ void swift::checkExplicitAvailability(Decl *decl) {
   // Skip if the command line option was not set and
   // accessors as we check the pattern binding decl instead.
   auto &ctx = decl->getASTContext();
-  auto DiagLevel = ctx.LangOpts.RequireExplicitAvailability;
-  if (!DiagLevel || isa<AccessorDecl>(decl))
+  if (isa<AccessorDecl>(decl))
     return;
 
+  DiagnosticBehavior DiagLevel;
+  switch (ctx.LangOpts.RequireExplicitAvailabilityBehavior) {
+  case LangOptions::RequireExplicitAvailabilityDiagnosticBehavior::Ignore:
+    return;
+  case LangOptions::RequireExplicitAvailabilityDiagnosticBehavior::Warning:
+    DiagLevel = DiagnosticBehavior::Warning;
+    break;
+  case LangOptions::RequireExplicitAvailabilityDiagnosticBehavior::Error:
+    DiagLevel = DiagnosticBehavior::Error;
+    break;
+  }
+
   // Only look at decls at module level or in extensions.
   // This could be changed to force having attributes on all decls.
   if (!decl->getDeclContext()->isModuleScopeContext() &&
@@ -4695,7 +4706,7 @@ void swift::checkExplicitAvailability(Decl *decl) {
 
   if (declNeedsExplicitAvailability(decl)) {
     auto diag = decl->diagnose(diag::public_decl_needs_availability);
-    diag.limitBehavior(*DiagLevel);
+    diag.limitBehavior(DiagLevel);
 
     auto suggestPlatform = ctx.LangOpts.RequireExplicitAvailabilityTarget;
     if (!suggestPlatform.empty()) {