[clangd] Log a message when gRPC support is off, but remote-index is configured

[kadircet]

Summary

Before this change clangd would emit a diagnostic whenever remote-index
was configured but binary didn't have grpc support.

This can be annoying when projects are configuring remote-index through their
configs but developers have a clangd binary without the support.

[kadircet]

• Update the tests

[kbobyrev]

Thanks!

clang-tools-extra/clangd/ConfigCompile.cpp

void compile(Fragment::IndexBlock &&F) {
     if (F.External)
       compile(std::move(**F.External), F.External->Range);
   }
 
   void compile(Fragment::IndexBlock::ExternalBlock &&External,
                llvm::SMRange BlockRange) {
 #ifndef CLANGD_ENABLE_REMOTE
     if (External.Server) {
-      diag(Error, "Clangd isn't compiled with remote index support, ignoring "
-                  "Server." External.Server->Range);
+      elog("Clangd isn't compiled with remote index support, ignoring Server: "
+           "{0}",
+           *External.Server);
       External.Server.reset();
     }
 #endif
     // Make sure exactly one of the Sources is set.
     unsigned SourceCount =
         External.File.hasValue() + External.Server.hasValue();
     if (SourceCount != 1) {
       diag(Error, "Exactly one of File or Server must be set.", BlockRange);

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

 }
 
 TEST_F(ConfigCompileTests, ExternalBlockWarnOnMultipleSource) {
   Fragment::IndexBlock::ExternalBlock External;
   External.File.emplace("");
   External.Server.emplace("");
   Frag.Index.External = std::move(External);
   compileAndApply();
-  llvm::StringLiteral ExpectedDiag =
 #ifdef CLANGD_ENABLE_REMOTE
-      "Exactly one of File or Server must be set.";
+  EXPECT_THAT(
+      Diags.Diagnostics,
+      Contains(AllOf(DiagMessage("Exactly one of File or Server must be set."),
+                     DiagKind(llvm::SourceMgr::DK_Error))));
 #else
-      "Clangd isn't compiled with remote index support, ignoring Server.";
+  ASSERT_TRUE(Conf.Index.External.hasValue());
+  EXPECT_EQ(Conf.Index.External->Kind, Config::ExternalIndexSpec::File);
 #endif
-  EXPECT_THAT(Diags.Diagnostics,
-              Contains(AllOf(DiagMessage(ExpectedDiag),
-                             DiagKind(llvm::SourceMgr::DK_Error))));
 }
 
 TEST_F(ConfigCompileTests, ExternalBlockErrOnNoSource) {
   Frag.Index.External.emplace(Fragment::IndexBlock::ExternalBlock{});
   compileAndApply();
   EXPECT_THAT(
       Diags.Diagnostics,
       Contains(AllOf(DiagMessage("Exactly one of File or Server must be set."),