[Serialization] Allow clients to deserialize all generic signatures.

As a debugging aid, allow clients to deserialize all generic signatures
in a given module file.

Author: DougGregor

include/swift/AST/Module.h

@@ -1173,6 +1173,15 @@ class LoadedFile : public FileUnit {
 
   virtual bool isSystemModule() const { return false; }
 
+  /// Retrieve the set of generic signatures stored within this module.
+  ///
+  /// \returns \c true if this module file supports retrieving all of the
+  /// generic signatures, \c false otherwise.
+  virtual bool getAllGenericSignatures(
+                 SmallVectorImpl<GenericSignature*> &genericSignatures) {
+    return false;
+  }
+
   static bool classof(const FileUnit *file) {
     return file->getKind() == FileUnitKind::SerializedAST ||
            file->getKind() == FileUnitKind::ClangModule;

include/swift/Serialization/SerializedModuleLoader.h

@@ -195,6 +195,10 @@ class SerializedASTFile final : public LoadedFile {
 
   virtual const clang::Module *getUnderlyingClangModule() const override;
 
+  virtual bool getAllGenericSignatures(
+                   SmallVectorImpl<GenericSignature*> &genericSignatures)
+                override;
+
   static bool classof(const FileUnit *file) {
     return file->getKind() == FileUnitKind::SerializedAST;
   }

lib/Serialization/ModuleFile.cpp

@@ -2175,6 +2175,17 @@ bool SerializedASTFile::hasEntryPoint() const {
   return File.Bits.HasEntryPoint;
 }
 
+bool SerializedASTFile::getAllGenericSignatures(
+                       SmallVectorImpl<GenericSignature*> &genericSignatures) {
+  genericSignatures.clear();
+  for (unsigned index : indices(File.GenericSignatures)) {
+    if (auto genericSig = File.getGenericSignature(index + 1))
+      genericSignatures.push_back(genericSig);
+  }
+
+  return true;
+}
+
 ClassDecl *SerializedASTFile::getMainClass() const {
   assert(hasEntryPoint());
   return cast_or_null<ClassDecl>(File.getDecl(File.Bits.EntryPointDeclID));