AST/Sema: Resolve AvailabilityDomain in SemanticAvailableAttrRequest.

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

include/swift/AST/Attr.h

@@ -912,6 +912,12 @@ private:
 private:
   friend class SemanticAvailableAttrRequest;
 
+  void setCachedDomain(AvailabilityDomain domain) {
+    assert(!Bits.AvailableAttr.HasDomain);
+    Domain = domain;
+    Bits.AvailableAttr.HasDomain = true;
+  }
+
   bool hasComputedSemanticAttr() const {
     return Bits.AvailableAttr.HasComputedSemanticAttr;
   }

include/swift/AST/AvailabilityDomain.h

@@ -140,6 +140,10 @@ public:
     return AvailabilityDomain(Kind::Embedded);
   }
 
+  /// Returns the built-in availability domain identified by the given string.
+  static std::optional<AvailabilityDomain>
+  builtinDomainForString(StringRef string);
+
   Kind getKind() const {
     if (auto inlineDomain = getInlineDomain())
       return inlineDomain->getKind();