[Tooling] Add a isSingleProcess() helper to ToolExecutor

Summary:
Used in clangd's symbol builder to optimize for the common
shared-memory executor case.

Reviewers: ioeric

Reviewed By: ioeric

Subscribers: kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D51164

llvm-svn: 340599
diff --git a/clang/unittests/Tooling/ExecutionTest.cpp b/clang/unittests/Tooling/ExecutionTest.cpp
index 26db8c6..e5dc98d 100644
--- a/clang/unittests/Tooling/ExecutionTest.cpp
+++ b/clang/unittests/Tooling/ExecutionTest.cpp
@@ -96,6 +96,8 @@
 
   StringRef getExecutorName() const override { return ExecutorName; }
 
+  bool isSingleProcess() const override { return true; }
+
   llvm::Error
   execute(llvm::ArrayRef<std::pair<std::unique_ptr<FrontendActionFactory>,
                                    ArgumentsAdjuster>>) override {