[clang] [clang-tools-extra] [clang-tidy] Do not crash when an empty directory is used in the comp… (PR #156873)

[Victor Chernyakin via cfe-commits]

================
@@ -574,22 +574,30 @@ int ClangTool::run(ToolAction *Action) {
       continue;
     }
     for (CompileCommand &CompileCommand : CompileCommandsForFile) {
+      // If the 'directory' field of the compilation database is empty, display
+      // an error and use the working directory instead.
+      std::string Directory = CompileCommand.Directory;
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localspook wrote:

```suggestion
      StringRef Directory = CompileCommand.Directory;
```
?

https://github.com/llvm/llvm-project/pull/156873