[clangd] Try to fix windows buildbot. NFC

http://45.33.8.238/win/19116/step_9.txt
diff --git a/clang-tools-extra/clangd/unittests/BackgroundIndexTests.cpp b/clang-tools-extra/clangd/unittests/BackgroundIndexTests.cpp
index 352a58b..cb4d23e 100644
--- a/clang-tools-extra/clangd/unittests/BackgroundIndexTests.cpp
+++ b/clang-tools-extra/clangd/unittests/BackgroundIndexTests.cpp
@@ -144,8 +144,10 @@
       Context::empty(), FS, CDB, [&](llvm::StringRef) { return &MSS; },
       /*ThreadPoolSize=*/4, /*OnProgress=*/nullptr, std::move(ContextProvider));
   // Index the two files.
-  for (auto &Cmd : Cmds)
-    CDB.setCompileCommand(testPath(Cmd.Filename), std::move(Cmd));
+  for (auto &Cmd : Cmds) {
+    std::string FullPath = testPath(Cmd.Filename);
+    CDB.setCompileCommand(FullPath, std::move(Cmd));
+  }
   // Wait for both files to be indexed.
   ASSERT_TRUE(Idx.blockUntilIdleForTest());
   EXPECT_THAT(runFuzzyFind(Idx, ""),