[clangd][nfc] Show more information in logs when compiler instance pr…

…epare fails

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 clangd/clangd#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'
```

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D104056

clang-tools-extra/clangd/ParsedAST.cpp

@@ -289,8 +289,15 @@ ParsedAST::build(llvm::StringRef Filename, const ParseInputs &Inputs,
       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];

clang-tools-extra/clangd/Preamble.cpp

@@ -390,8 +390,8 @@ buildPreamble(PathRef FileName, CompilerInvocation CI,
         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;
   }
 }