[AST] Introduce explicit actions for ASTWalker

Replace the use of bool and pointer returns for
`walkToXXXPre`/`walkToXXXPost`, and instead use
explicit actions such as `Action::Continue(E)`,
`Action::SkipChildren(E)`, and `Action::Stop()`.
There are also conditional variants, e.g
`Action::SkipChildrenIf`, `Action::VisitChildrenIf`,
and `Action::StopIf`.

There is still more work that can be done here, in
particular:

- SourceEntityWalker still needs to be migrated.
- Some uses of `return false` in pre-visitation
methods can likely now be replaced by
`Action::Stop`.
- We still use bool and pointer returns internally
within the ASTWalker traversal, which could likely
be improved.

But I'm leaving those as future work for now as
this patch is already large enough.

lib/Serialization/SerializeDoc.cpp

@@ -413,45 +413,47 @@ static void writeDeclCommentTable(
                          SourceOrder++ });
     }
 
-    bool walkToDeclPre(Decl *D) override {
+    PreWalkAction walkToDeclPre(Decl *D) override {
       if (!shouldIncludeDecl(D, /*ExcludeDoubleUnderscore*/true))
-        return false;
+        return Action::SkipChildren();
       if (!shouldSerializeDoc(D))
-        return true;
+        return Action::Continue();
       if (auto *ED = dyn_cast<ExtensionDecl>(D)) {
         writeDocForExtensionDecl(ED);
-        return true;
+        return Action::Continue();
       }
 
       auto *VD = dyn_cast<ValueDecl>(D);
       if (!VD)
-        return true;
+        return Action::Continue();
 
       // Compute USR.
       {
         USRBuffer.clear();
         llvm::raw_svector_ostream OS(USRBuffer);
         if (ide::printValueDeclUSR(VD, OS))
-          return true;
+          return Action::Continue();
       }
 
       generator.insert(copyString(USRBuffer.str()),
                        { VD->getBriefComment(), D->getRawComment(),
                          GroupContext.getGroupSequence(VD),
                          SourceOrder++ });
-      return true;
+      return Action::Continue();
     }
 
-    std::pair<bool, Stmt *> walkToStmtPre(Stmt *S) override {
-      return { false, S };
+    PreWalkResult<Stmt *> walkToStmtPre(Stmt *S) override {
+      return Action::SkipChildren(S);
     }
-
-    std::pair<bool, Expr *> walkToExprPre(Expr *E) override {
-      return { false, E };
+    PreWalkResult<Expr *> walkToExprPre(Expr *E) override {
+      return Action::SkipChildren(E);
+    }
+    PreWalkAction walkToTypeReprPre(TypeRepr *T) override {
+      return Action::SkipChildren();
+    }
+    PreWalkAction walkToParameterListPre(ParameterList *PL) override {
+      return Action::SkipChildren();
     }
-
-    bool walkToTypeReprPre(TypeRepr *T) override { return false; }
-    bool walkToParameterListPre(ParameterList *PL) override { return false; }
   };
 
   DeclCommentTableWriter Writer(GroupContext);
@@ -692,10 +694,18 @@ struct BasicDeclLocsTableWriter : public ASTWalker {
                            FWriter(FWriter),
                            DocWriter(DocWriter) {}
 
-  std::pair<bool, Stmt *> walkToStmtPre(Stmt *S) override { return { false, S };}
-  std::pair<bool, Expr *> walkToExprPre(Expr *E) override { return { false, E };}
-  bool walkToTypeReprPre(TypeRepr *T) override { return false; }
-  bool walkToParameterListPre(ParameterList *PL) override { return false; }
+  PreWalkResult<Stmt *> walkToStmtPre(Stmt *S) override {
+    return Action::SkipChildren(S);
+  }
+  PreWalkResult<Expr *> walkToExprPre(Expr *E) override {
+    return Action::SkipChildren(E);
+  }
+  PreWalkAction walkToTypeReprPre(TypeRepr *T) override {
+    return Action::SkipChildren();
+  }
+  PreWalkAction walkToParameterListPre(ParameterList *PL) override {
+    return Action::SkipChildren();
+  }
 
   Optional<uint32_t> calculateNewUSRId(Decl *D) {
     llvm::SmallString<512> Buffer;
@@ -711,24 +721,24 @@ struct BasicDeclLocsTableWriter : public ASTWalker {
     return true;
   }
 
-  bool walkToDeclPre(Decl *D) override {
+  PreWalkAction walkToDeclPre(Decl *D) override {
     // .swiftdoc doesn't include comments for double underscored symbols, but
     // for .swiftsourceinfo, having the source location for these symbols isn't
     // a concern because these symbols are in .swiftinterface anyway.
     if (!shouldIncludeDecl(D, /*ExcludeDoubleUnderscore*/false))
-      return false;
+      return Action::SkipChildren();
     if (!shouldSerializeSourceLoc(D))
-      return true;
+      return Action::Continue();
 
     auto *File = D->getDeclContext()->getModuleScopeContext();
     auto RawLocs = cast<FileUnit>(File)->getExternalRawLocsForDecl(D);
     if (!RawLocs.hasValue())
-      return true;
+      return Action::Continue();
 
     // If we have handled this USR before, don't proceed
     auto USR = calculateNewUSRId(D);
     if (!USR.hasValue())
-      return true;
+      return Action::Continue();
 
     llvm::SmallString<128> AbsolutePath = RawLocs->SourceFilePath;
     llvm::sys::fs::make_absolute(AbsolutePath);
@@ -742,7 +752,7 @@ struct BasicDeclLocsTableWriter : public ASTWalker {
     writeRawLoc(RawLocs->StartLoc, Writer, FWriter);
     writeRawLoc(RawLocs->EndLoc, Writer, FWriter);
 
-    return true;
+    return Action::Continue();
   }
 };