AST: Make some ASTContext parameters const-correct.

Author: tshortli

include/swift/AST/Attr.h

@@ -3222,7 +3222,7 @@ class SemanticAvailableAttr final {
 
   /// Returns the effective range in which the declaration with this attribute
   /// was introduced.
-  AvailabilityRange getIntroducedRange(ASTContext &Ctx) const;
+  AvailabilityRange getIntroducedRange(const ASTContext &Ctx) const;
 
   /// The version tuple written in source for the `deprecated:` component.
   std::optional<llvm::VersionTuple> getDeprecated() const {

include/swift/AST/AvailabilityDomain.h

@@ -88,7 +88,7 @@ class AvailabilityDomain final {
 
   /// Returns true if this domain is considered active in the current
   /// compilation context.
-  bool isActive(ASTContext &ctx) const;
+  bool isActive(const ASTContext &ctx) const;
 
   /// Returns the string to use in diagnostics to identify the domain. May
   /// return an empty string.

lib/AST/Availability.cpp

@@ -828,10 +828,13 @@ bool AvailabilityInference::isAvailableAsSPI(const Decl *D) {
 }
 
 AvailabilityRange
-SemanticAvailableAttr::getIntroducedRange(ASTContext &Ctx) const {
-  auto *attr = getParsedAttr();
+SemanticAvailableAttr::getIntroducedRange(const ASTContext &Ctx) const {
   assert(domain.isActive(Ctx));
 
+  auto *attr = getParsedAttr();
+  if (!attr->Introduced.has_value())
+    return AvailabilityRange::alwaysAvailable();
+
   llvm::VersionTuple IntroducedVersion = attr->Introduced.value();
   StringRef Platform;
   llvm::VersionTuple RemappedIntroducedVersion;

lib/AST/AvailabilityDomain.cpp

@@ -16,7 +16,7 @@
 
 using namespace swift;
 
-bool AvailabilityDomain::isActive(ASTContext &ctx) const {
+bool AvailabilityDomain::isActive(const ASTContext &ctx) const {
   switch (kind) {
   case Kind::Universal:
   case Kind::SwiftLanguage: