[clangd] Fix diagnostic errors in the test code, NFC.

[ilya-biryukov]

unittests/clangd/XRefsTests.cpp

599–601

I would go with #pragma once in the test code, it adds less noise. WDYT?

[hokein]

Use #program once

unittests/clangd/XRefsTests.cpp

599–601

I thought #pragma once is non-standard, but the diagnostic message also suggest this fix. Changed to it.

[ilya-biryukov]

unittests/clangd/XRefsTests.cpp

599–601

Thanks! It's non-standard, but supported on every modern compiler, so we should be fine :-)

unittests/clangd/XRefsTests.cpp

TEST(GoToInclude, All) {
   int foo;
   #in$5^clude "$6^foo.h"$7^
   )cpp";
   Annotations SourceAnnotations(SourceContents);
   FS.Files[FooCpp] = SourceAnnotations.code();
   auto FooH = testPath("foo.h");
   auto FooHUri = URIForFile{FooH};
 
-  const char *HeaderContents = R"cpp([[]]int a;)cpp";
+  const char *HeaderContents = R"cpp([[]]#pragma once
+                                     int a;
+                                     )cpp";
   Annotations HeaderAnnotations(HeaderContents);
   FS.Files[FooH] = HeaderAnnotations.code();
 
   Server.addDocument(FooH, HeaderAnnotations.code());
   Server.addDocument(FooCpp, SourceAnnotations.code());
 
   // Test include in preamble.
   auto Locations =