AST/Sema: Sink AvailabilityScopeBuilder from Sema to AST.

Author: tshortli

include/swift/AST/AvailabilityScope.h

@@ -185,6 +185,10 @@ class AvailabilityScope : public ASTAllocated<AvailabilityScope> {
                     SourceRange SrcRange, const AvailabilityContext Info);
 
 public:
+  /// Constructs the root availability scope for the given file and builds out
+  /// the scope tree for the top level contents of the file.
+  static AvailabilityScope *getOrBuildForSourceFile(SourceFile &SF);
+
   /// Create the root availability scope for the given SourceFile.
   static AvailabilityScope *createForSourceFile(SourceFile *SF,
                                                 const AvailabilityContext Info);

include/swift/AST/SourceFile.h

@@ -776,7 +776,7 @@ class SourceFile final : public FileUnit {
 
   /// Get the root availability scope for the file. The root scope may be
   /// null if the scope tree has not been built yet. Use
-  /// TypeChecker::getOrBuildAvailabilityScope() to get a built
+  /// `AvailabilityScope::getOrBuildForSourceFile()` to get a built
   /// root of the tree.
   AvailabilityScope *getAvailabilityScope() const;