[NFC] [clangd] [Modules] Logging more if compilation failed

Coming from internal using experience.

The original log is just confusing... Users can't understand it unless
he reads the code.

This patch tries to make this more understandable.

Note that the mentioned module files in the logs might be removed
intentionally. We need another option to allow users to remain these
module files. Let's done this in another patch.

Author: ChuanqiXu9

clang-tools-extra/clangd/ModulesBuilder.cpp

@@ -160,6 +160,16 @@ class ReusablePrerequisiteModules : public PrerequisiteModules {
           RequiredModule->getModuleFilePath().str());
   }
 
+  std::string getAsString() const {
+    std::string Result;
+    llvm::raw_string_ostream OS(Result);
+    for (const auto &ModuleFile : RequiredModules) {
+      OS << "-fmodule-file=" << ModuleFile->getModuleName() << "="
+         << ModuleFile->getModuleFilePath() << " ";
+    }
+    return Result;
+  }
+
   bool canReuse(const CompilerInvocation &CI,
                 llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem>) const override;
 
@@ -296,8 +306,27 @@ buildModuleFile(llvm::StringRef ModuleName, PathRef ModuleUnitFileName,
   GenerateReducedModuleInterfaceAction Action;
   Clang->ExecuteAction(Action);
 
-  if (Clang->getDiagnostics().hasErrorOccurred())
-    return llvm::createStringError("Compilation failed");
+  if (Clang->getDiagnostics().hasErrorOccurred()) {
+    std::string Cmds;
+    for (const auto &Arg : Inputs.CompileCommand.CommandLine) {
+      if (!Cmds.empty())
+        Cmds += " ";
+      Cmds += Arg;
+    }
+
+    clangd::vlog("Failed to compile {0} with command: {1}.", ModuleUnitFileName,
+                 Cmds);
+
+    std::string BuiltModuleFilesStr = BuiltModuleFiles.getAsString();
+    if (!BuiltModuleFilesStr.empty())
+      clangd::vlog("The actual used module files built by clangd is {0}",
+                   BuiltModuleFilesStr);
+
+    return llvm::createStringError(
+        llvm::formatv("Failed to compile {0}. Use '--log=verbose' to view "
+                      "detailed failure reasons.",
+                      ModuleUnitFileName));
+  }
 
   return ModuleFile{ModuleName, Inputs.CompileCommand.Output};
 }