[clangd][nfc] Show more information in logs when compiler instance prepare fails

[ArcsinX]

Summary

Without this patch clangd silently process compiler instance prepare failure and only LSP errors "Invalid AST" could be found in logs.
E.g. the reason of the problem https://github.com/clangd/clangd/issues/734 is impossible to understand without verbose logs or with disabled background index.
This patch adds more information into logs to help understand the reason of such failures.

Logs without this patch:

E[...] Could not build a preamble for file test.cpp version 1

Logs with this patch:

E[...] Could not build a preamble for file test.cpp version 1: CreateTargetInfo() return null
..
E[...] Failed to prepare a compiler instance: unknown target ABI 'lp64'

[sammccall]

Thanks, LGTM

Sorry about the delay - we got sidetracked into some ideas about whether these errors should actually be surfaced to the user as diagnostics instead. But that shouldn't block this patch.

clang-tools-extra/clangd/ParsedAST.cpp

296

there's a bunch of logic that determines which diags we actually end up storing.

I'd suggest always calling take() and testing whether the result is empty, rather than using getNumErrors() which bypasses all our logic.

Luckily, we do (always?) end up storing the relevant diags here, assuming they're errors without a source location attached.
However I don't think we should hard-code that assumption here.

[ArcsinX]

Check ASTDiags.take() for empty instead of ASTDiags.getNumErrors() value check

clang-tools-extra/clangd/ParsedAST.cpp

if (Preamble) {
     Patch = PreamblePatch::create(Filename, Inputs, *Preamble);
     Patch->apply(*CI);
     PreserveDiags = Patch->preserveDiagnostics();
   }
   auto Clang = prepareCompilerInstance(
       std::move(CI), PreamblePCH,
       llvm::MemoryBuffer::getMemBufferCopy(Inputs.Contents, Filename), VFS,
       ASTDiags);
-  if (!Clang)
+  if (!Clang) {
+    // The last diagnostic contains information about the reason of this
+    // failure.
+    std::vector<Diag> Diags(ASTDiags.take());
+    elog("Failed to prepare a compiler instance: {0}",
+         !Diags.empty() ? static_cast<DiagBase &>(Diags.back()).Message
+                        : "unknown error");
     return None;
+  }
 
   auto Action = std::make_unique<ClangdFrontendAction>();
   const FrontendInputFile &MainInput = Clang->getFrontendOpts().Inputs[0];
   if (!Action->BeginSourceFile(*Clang, MainInput)) {
     log("BeginSourceFile() failed when building AST for {0}",
         MainInput.getFile());
     return None;
   }

clang-tools-extra/clangd/Preamble.cpp

vlog("Built preamble of size {0} for file {1} version {2}",
          BuiltPreamble->getSize(), FileName, Inputs.Version);
     std::vector<Diag> Diags = PreambleDiagnostics.take();
     return std::make_shared<PreambleData>(
         Inputs, std::move(*BuiltPreamble), std::move(Diags),
         SerializedDeclsCollector.takeIncludes(),
         SerializedDeclsCollector.takeMacros(), std::move(StatCache),
         SerializedDeclsCollector.takeCanonicalIncludes());
   } else {
-    elog("Could not build a preamble for file {0} version {1}", FileName,
-         Inputs.Version);
+    elog("Could not build a preamble for file {0} version {1}: {2}", FileName,
+         Inputs.Version, BuiltPreamble.getError().message());
     return nullptr;
   }
 }
 
 bool isPreambleCompatible(const PreambleData &Preamble,
                           const ParseInputs &Inputs, PathRef FileName,
                           const CompilerInvocation &CI) {
   auto ContentsBuffer =