[clangd] Add server capability advertising hot-reloading of CDBs.

[sammccall]

Summary

Currently some clients watch for CDB changes and restart clangd, now that we
can reload compile_commands.json ourselves this is counterproductive.
The capability allows this behavior to be phased out.

This is going to be a mild regression, as we do not actually watch for files on
disk and so new diagnostics need to wait until a rebuild is requested e.g. due
to file change (and the internal caches have expired).
However this is still a better tradeoff (and if it's important, we can request
the client to watch files for us in the future).

clang-tools-extra/clangd/ClangdLSPServer.cpp

llvm::json::Object Result{
                  {"resolveProvider", false},
              }},
             {"hoverProvider", true},
             {"renameProvider", std::move(RenameProvider)},
             {"selectionRangeProvider", true},
             {"documentSymbolProvider", true},
             {"workspaceSymbolProvider", true},
             {"referencesProvider", true},
-            {"astProvider", true},
+            {"astProvider", true}, // clangd extension
             {"executeCommandProvider",
              llvm::json::Object{
                  {"commands",
                   {ExecuteCommandParams::CLANGD_APPLY_FIX_COMMAND,
                    ExecuteCommandParams::CLANGD_APPLY_TWEAK}},
              }},
             {"typeHierarchyProvider", true},
-            {"memoryUsageProvider", true}, // clangd extension.
+            {"memoryUsageProvider", true}, // clangd extension
+            {"compilationDatabase",        // clangd extension
+             llvm::json::Object{{"automaticReload", true}}},
             {"callHierarchyProvider", true},
         }}}};
   if (Opts.Encoding)
     Result["offsetEncoding"] = *Opts.Encoding;
   if (Opts.TheiaSemanticHighlighting)
     Result.getObject("capabilities")
         ->insert(
             {"semanticHighlighting",

clang-tools-extra/clangd/test/initialize-params.test

 # RUN: clangd -lit-test < %s | FileCheck -strict-whitespace %s
 # Test initialize request parameters with rootUri
 {"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":123,"rootUri":"test:///workspace","capabilities":{},"trace":"off"}}
 #      CHECK:  "id": 0,
 # CHECK-NEXT:  "jsonrpc": "2.0",
 # CHECK-NEXT:  "result": {
 # CHECK-NEXT:    "capabilities": {
 # CHECK-NEXT:      "astProvider": true,
 # CHECK-NEXT:      "callHierarchyProvider": true,
 # CHECK-NEXT:      "codeActionProvider": true,
+# CHECK-NEXT:      "compilationDatabase": {
+# CHECK-NEXT:        "automaticReload": true
+# CHECK-NEXT:      },
 # CHECK-NEXT:      "completionProvider": {
 # CHECK-NEXT:        "allCommitCharacters": [
 # CHECK-NEXT:          " ",
 # CHECK-NEXT:          "\t",
 # CHECK-NEXT:          "(",
 # CHECK-NEXT:          ")",
 # CHECK-NEXT:          "[",
 # CHECK-NEXT:          "]",