[clangd] Move inserted include from detail -> documentation.

[sammccall]

Summary

Many clients try to display all the detail inline, with poor results.

[hokein]

clang-tools-extra/clangd/CodeComplete.cpp

1834

#include => From, IIUC discussion from the chat.

maybe worth a comment explaining why we make such a decision, the description seems good enough.

[sammccall]

This needed a nontrivial merge after D79157, because we need to concatenate markup::Documents. Don't think there's anything surprising in there so going ahead, happy to address any comments though.

clang-tools-extra/clangd/CodeComplete.cpp

LSP.label = ((InsertInclude && InsertInclude->Insertion)
               RequiredQualifier + Name + Signature;
 
   LSP.kind = Kind;
   LSP.detail = BundleSize > 1
                    ? std::string(llvm::formatv("[{0} overloads]", BundleSize))
                    : ReturnType;
   LSP.deprecated = Deprecated;
   if (InsertInclude)
-    LSP.detail += "\n" + InsertInclude->Header;
-  LSP.documentation = Documentation;
+    LSP.documentation = llvm::formatv("#include {0}", InsertInclude->Header);
+  if (!Documentation.empty()) {
+    if (!LSP.documentation.empty())
+      LSP.documentation += "\n";
+    LSP.documentation += Documentation;
+  }
   LSP.sortText = sortText(Score.Total, Name);
   LSP.filterText = Name;
   LSP.textEdit = {CompletionTokenRange, RequiredQualifier + Name};
   // Merge continuous additionalTextEdits into main edit. The main motivation
   // behind this is to help LSP clients, it seems most of them are confused when
   // they are provided with additionalTextEdits that are consecutive to main
   // edit.
   // Note that we store additional text edits from back to front in a line. That

clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp

TEST(CompletionTest, Render) {
   Opts.IncludeIndicator.NoInsert = "";
   Opts.EnableSnippets = false;
 
   auto R = C.render(Opts);
   EXPECT_EQ(R.label, "Foo::x(bool) const");
   EXPECT_EQ(R.insertText, "Foo::x");
   EXPECT_EQ(R.insertTextFormat, InsertTextFormat::PlainText);
   EXPECT_EQ(R.filterText, "x");
-  EXPECT_EQ(R.detail, "int\n\"foo.h\"");
-  EXPECT_EQ(R.documentation, "This is x().");
+  EXPECT_EQ(R.detail, "int");
+  EXPECT_EQ(R.documentation, "#include \"foo.h\"\nThis is x().");
   EXPECT_THAT(R.additionalTextEdits, IsEmpty());
   EXPECT_EQ(R.sortText, sortText(1.0, "x"));
   EXPECT_FALSE(R.deprecated);
   EXPECT_EQ(R.score, .5f);
 
   Opts.EnableSnippets = true;
   R = C.render(Opts);
   EXPECT_EQ(R.insertText, "Foo::x(${0:bool})");
   EXPECT_EQ(R.insertTextFormat, InsertTextFormat::Snippet);
 
   Include.Insertion.emplace();
   R = C.render(Opts);
   EXPECT_EQ(R.label, "^Foo::x(bool) const");
   EXPECT_THAT(R.additionalTextEdits, Not(IsEmpty()));
 
   Opts.ShowOrigins = true;
   R = C.render(Opts);
   EXPECT_EQ(R.label, "^[AS]Foo::x(bool) const");
 
   C.BundleSize = 2;
   R = C.render(Opts);
-  EXPECT_EQ(R.detail, "[2 overloads]\n\"foo.h\"");
+  EXPECT_EQ(R.detail, "[2 overloads]");
+  EXPECT_EQ(R.documentation, "#include \"foo.h\"\nThis is x().");
 
   C.Deprecated = true;
   R = C.render(Opts);
   EXPECT_TRUE(R.deprecated);
 }
 
 TEST(CompletionTest, IgnoreRecoveryResults) {
   auto Results = completions(