AST/Sema: Resolve AvailabilityDomain in SemanticAvailableAttrRequest.

Look up the AvailabilityDomain given its name during type checking, instead of
parsing.

lib/AST/AvailabilityDomain.cpp

@@ -13,9 +13,28 @@
 #include "swift/AST/AvailabilityDomain.h"
 #include "swift/AST/ASTContext.h"
 #include "swift/AST/Decl.h"
+#include "llvm/ADT/StringSwitch.h"
 
 using namespace swift;
 
+std::optional<AvailabilityDomain>
+AvailabilityDomain::builtinDomainForString(StringRef string) {
+  auto domain = llvm::StringSwitch<std::optional<AvailabilityDomain>>(string)
+                    .Case("*", AvailabilityDomain::forUniversal())
+                    .Case("swift", AvailabilityDomain::forSwiftLanguage())
+                    .Case("_PackageDescription",
+                          AvailabilityDomain::forPackageDescription())
+                    .Default(std::nullopt);
+
+  if (domain)
+    return domain;
+
+  if (auto platformKind = platformFromString(string))
+    return AvailabilityDomain::forPlatform(*platformKind);
+
+  return std::nullopt;
+}
+
 bool AvailabilityDomain::isActive(const ASTContext &ctx) const {
   switch (getKind()) {
   case Kind::Universal: