[clangd] Fix "fail to create file URI" warnings in FileIndexTest.

[hokein]

Summary

When running the FileIndexTest, it shows "Failed to create an URI
for file XXX: not a valid absolute file path" warnings, and the
generated Symbols is missing Location information.

[ioeric]

lg

unittests/clangd/FileIndexTests.cpp

llvm::Optional<ParsedAST> build(llvm::StringRef BasePath,
                                 llvm::StringRef Code) {
   if (Code.empty())
     return llvm::None;
 
   assert(llvm::sys::path::extension(BasePath).empty() &&
          "BasePath must be a base file path without extension.");
   llvm::IntrusiveRefCntPtr<vfs::InMemoryFileSystem> VFS(
       new vfs::InMemoryFileSystem);
-  std::string Path = (BasePath + ".cpp").str();
-  std::string Header = (BasePath + ".h").str();
+  std::string Path = testPath((BasePath + ".cpp").str());
+  std::string Header = testPath((BasePath + ".h").str());
   VFS->addFile(Path, 0, llvm::MemoryBuffer::getMemBuffer(""));
   VFS->addFile(Header, 0, llvm::MemoryBuffer::getMemBuffer(Code));
   const char *Args[] = {"clang", "-xc++", "-include", Header.c_str(),
                         Path.c_str()};
 
   auto CI = createInvocationFromCommandLine(Args);
 
   auto Buf = llvm::MemoryBuffer::getMemBuffer(Code);