AST: Add a DeclContext to AvailabilityDomain::builtinDomainForString().

A context is needed to customize the behavior of lookup in downstream versions
of Swift.

Author: tshortli

include/swift/AST/AvailabilityDomain.h

@@ -26,6 +26,7 @@
 
 namespace swift {
 class ASTContext;
+class DeclContext;
 
 /// Represents a dimension of availability (e.g. macOS platform or Swift
 /// language mode).
@@ -142,7 +143,7 @@ class AvailabilityDomain final {
 
   /// Returns the built-in availability domain identified by the given string.
   static std::optional<AvailabilityDomain>
-  builtinDomainForString(StringRef string);
+  builtinDomainForString(StringRef string, const DeclContext *declContext);
 
   Kind getKind() const {
     if (auto inlineDomain = getInlineDomain())

lib/AST/AvailabilityDomain.cpp

@@ -18,7 +18,11 @@
 using namespace swift;
 
 std::optional<AvailabilityDomain>
-AvailabilityDomain::builtinDomainForString(StringRef string) {
+AvailabilityDomain::builtinDomainForString(StringRef string,
+                                           const DeclContext *declContext) {
+  // This parameter is used in downstream forks, do not remove.
+  (void)declContext;
+
   auto domain = llvm::StringSwitch<std::optional<AvailabilityDomain>>(string)
                     .Case("*", AvailabilityDomain::forUniversal())
                     .Case("swift", AvailabilityDomain::forSwiftLanguage())

lib/Sema/TypeCheckAttr.cpp

@@ -8284,7 +8284,8 @@ SemanticAvailableAttrRequest::evaluate(swift::Evaluator &evaluator,
 
     // Attempt to resolve the domain specified for the attribute and diagnose
     // if no domain is found.
-    domain = AvailabilityDomain::builtinDomainForString(*string);
+    auto declContext = decl->getInnermostDeclContext();
+    domain = AvailabilityDomain::builtinDomainForString(*string, declContext);
     if (!domain) {
       if (auto suggestion = closestCorrectedPlatformString(*string)) {
         diags