[clangd] Log diagnostics if we failed to create a preamble.

[sammccall]

Summary

Really we want these to be shown to the client, but the path to do so involves
storing them in

Bandaid for https://github.com/clangd/clangd/issues/1408
See https://github.com/clangd/clangd/issues/1399 for motivation

clang-tools-extra/clangd/Preamble.cpp

if (BuiltPreamble) {
     Result->CanonIncludes = CapturedInfo.takeCanonicalIncludes();
     Result->StatCache = std::move(StatCache);
     Result->MainIsIncludeGuarded = CapturedInfo.isMainFileIncludeGuarded();
     return Result;
   }
 
   elog("Could not build a preamble for file {0} version {1}: {2}", FileName,
        Inputs.Version, BuiltPreamble.getError().message());
+  for (const Diag &D : PreambleDiagnostics.take()) {
+    if (D.Severity < DiagnosticsEngine::Error)
+      continue;
+    // Not an ideal way to show errors, but better than nothing!
+    elog("  error: {0}", D.Message);
+  }
   return nullptr;
 }
 
 bool isPreambleCompatible(const PreambleData &Preamble,
                           const ParseInputs &Inputs, PathRef FileName,
                           const CompilerInvocation &CI) {
   auto ContentsBuffer =
       llvm::MemoryBuffer::getMemBuffer(Inputs.Contents, FileName);